How does killing bear consequences?

Why don't you kill? Maybe because you don't like to be killed either? It would be good if Nyom Daniel gives an answer here himself.

well approach like that: puthujjanas always do their things from greed, aversion or ignorance. greed and aversion typically refer to kama [opinions, thougths, ideas included], and ignorance to ''non pleasant and non dis pleasant'' feelings, like the non-puthujjana puts it https://www.accesstoinsight.org/tipitaka/mn/mn.044.than.html, or even better to equanimity, especially the 4th jhana. Typically puthujjanas knows nothing about non-kama,and killing ties up to clinging to sensuality [at this point puthujjanas love to talk about their moronic idea of ''survival instincts'' to ''make reasonable'' their toxic deeds and their binary thinking ''it is the other one or me'', ''it is a matter of life or death''] and that clinging to sensuality makes for a miserable life and clinging to sensuality is the only way to not get jhanas and the joy. Now, the puthujjanas will talk about building something with their life, in order to realize their pathetic fantasy of transcendence, precisely in order to counter the rut and misery of life, precisely because they have zero idea of what they are doing [no matter what they claim] besides following their desires and clinging to kama, to transmit something to '''futur generations'', a society, a lineage, a career and all their idiotic stuff that they build and claim they are good people for building their stuff. The easiest counter point is to make them say that sometimes all the stuff they build is a burden for them, so if it is a burden even one time, then it is not worth doing it [but puthujjanas have no problem with the idea of doing something that does not work, so if this does not work, still no worry, at least one of you two will get tired to talk about that and move on].

How can I explain to a non follower of the dhamma that killing living beings intentionally has mental and physical consequences?

The law of kamma is like the natural law of physics. For any action, there will be a re-action in return. If you throw a ball up in the air, sooner or later the ball will fall back down. Similary, if a person kills someone, he'll face the risk of the victim's loved ones to exact their vengeance upon him in return. In the case of hunters, one day their luck will run out and they'll die from the same kind of wounds they used to inflict upon their preys. And even in the event they don't get the exact kind of retribution, the natural law will always find a way to turn around and bite back when the time is right, in this life or the next, the killer's life will be cut short in countless other ways: freak accidents, incurable painful diseases, mental disorder leading to suicide, etc.

One perspective is that, by refraining from hurting/killing other living beings you're not actually abstaining from something, but rather enabling others to live their lives (and if one wishes to act even more skillfully, enabling others to a life in happiness and/or less/no suffering). The argument is that skillful conduct is conducive to your own - as well as others - wellbeing and peace of mind. In the suttas, this is considered a gift:

There is the case where a disciple of the noble ones, abandoning the taking of life, abstains from taking life. In doing so, he gives freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings. In giving freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings, he gains a share in limitless freedom from danger, freedom from animosity, and freedom from oppression.

https://www.accesstoinsight.org/tipitaka/an/an08/an08.039.than.html

Conversely, killing is considered an unskilled action leading to ill-being to yourself and others (obviously). If the metaphysical arguments about different realms/hell seems far fetched, one can look at them as metaphors for different states of mind/wellbeing:

When a warrior strives & exerts himself in battle, if others then strike him down & slay him while he is striving & exerting himself in battle, then with the breakup of the body, after death, he is reborn in the company of devas slain in battle,' that is his wrong view. Now, there are two destinations for a person with wrong view, I tell you: either hell or the animal womb.

https://www.accesstoinsight.org/tipitaka/sn/sn42/sn42.003.than.html

Lastly, and perhaps a bit offtopic: laypeoples notions about buddhism can be considered their own responsibility, not yours. In buddhist terms, how they perceive buddhism is their sankharas, and they ultimately owns the vipaka (consequences) according to whether their actions/notions are helpful or not. If they like to put your message to trial, the onus is on them to try it out or carry on with their lives as usual...

old hunter-gatherers, had no consequences nor problems with hunting

Early hunter-gathers' life expectancy was low. There was a lot of mortality due to various reasons, not necessarily from old age. Also, their quality of life was low. Partly maybe due to karmic consequences.

Probably in these societies also there as consciences of action. Though working of karma in hunting many not be readily apparent. Say if you attack another tribe or kill someone within one's tribe there were visible consequences in terms of retribution and justice. This again attributed to karma.

How can I explain to a non follower of the dhamma that killing living beings intentionally has mental and physical consequences?

You can explain in common terms:

• what goes around comes around
• one reaps what one sows
• things catch up with the person

Essentially this is what is mentioned below:

Short life

“Here, young brahmin, a certain woman or man kills living beings, cruel, bloody-handed, given to violence and killing, merciless to living beings.

Through such karma, thus accomplished, thus undertaken, with the body‟s breaking up, after death,one re-appears in a plane of misery, an evil destination, a lower realm, in hell.

And if, with the body‟s breaking up, after death, one does not re-appear in a plane of misery, an evil destination, a lower realm, in hell, if one returns to a human state, then wherever one is reborn, one is short-lived.

This, young brahmin, is the way leading to a short life, namely, one harms living beings, is cruel, bloody-handed, given to violence and killing, merciless to living beings.

But here, young brahmin, a certain woman or man has given up harming living beings, refrains from harming living beings, laid down rod and sword, conscientious, merciful, one dwells beneficial and compassionate to all living beings.

Through such karma, thus accomplished, thus undertaken, with the body‟s breaking up, after death, one re-appears in a state of joy, in a happy destination, in heaven.

And if, with the body‟s breaking up, after death, one does not re-appear in a state of joy, in a happy destination, in heaven., if one returns to a human state, then wherever one is reborn, one is long-lived.

This, young brahmin, is the way leading to a long life, namely, one has given up harming living beings, refrains from harming living beings, laid down rod and sword, conscientious, merciful, dwells beneficial and compassionate to all living beings.

2 Sickliness

Here, young brahmin, a certain woman or man habitually injures living beings with a clod of earth, with a stick, or with a sword.

Through such karma, thus accomplished, thus undertaken, with the body‟s breaking up, after death, one re-appears in a plane of misery, an evil destination, a lower realm, in hell.

And if, with the body‟s breaking up, after death, one does not re-appear in a plane of misery, an evil destination, a lower realm, in hell, if one returns to a human state, then wherever one is reborn, one is sickly [constantly ill].

This, young brahmin, is the way leading to sickliness, namely, one habitually injures beings with a clod of earth, with a stick, or with a sword.

But here, young brahmin, a certain woman or man is not of the nature of injuring beings, refrains from harming living beings, laid down rod and sword, conscientious, merciful, one dwells beneficial and compassionate to all living beings.

Through such karma, thus accomplished, thus undertaken, with the body‟s breaking up, after death, one re-appears in a state of joy, in a happy destination, in heaven.

And if, with the body‟s breaking up, after death, one does not re-appear in a state of joy, in a happy destination, in heaven, if one returns to a human state, then wherever one is reborn, one is healthy.

This, young brahmin, is the way leading to health, namely, one has given up harming living beings, refrains from harming living beings, laid down rod and sword, conscientious, merciful, dwells beneficial and compassionate to all living beings.

Cūla Kamma Vibhaṅga Sutta

Killing without necessity is never justifiable. In modern times for example, the majority of people are meat, diary & egg eaters, but they aren't aware of how the food is made & what the animals had to go through. Since humans can thrive without animal products, it's somewhard to justify supporting those industries.

Similarly, discrimination or killing isn't moral because one doesn't want to be discriminated or killed (due to arbitrary standards). So it's always best to put oneself in the shoes of the victim. Tell them. A lot of people pay lip service to compassion & never really practise it because it sounds like a nice concept.

There are internal consequences to be experienced & external conseaquences, although the latter is somewhat less under your influence as the former.

Ask them ;if the pursuit of happiness isn't the goal of beings.

When they admit that the pursuit of happiness is the goal of beings;

Ask them if there are not paths worthy of pursuit generally approved of by society and successful people?

When they admit that there are paths generally approved of as having good destinations.

Ask them then if it is generally held that any path leads to any destination or if it is generally held that specific paths have specific outcomes according to development?

When they admit that it is generally held that paths produce specific consequences according to development;

Ask them if paths are not developed by behavior, achieved and brought to culmination by the merit of actions of one pursuing the path?

When they admit that happiness is thus pursued by behavior and actions have specific consequences;

Here you can then say that there is a claim of highest happiness and that it has been proclaimed by the Buddha as the most worthy pursuit and that the action of killing is dispersive of it's attainment because it's attainment relies on development of sympathy.

You can say you think Buddha was the smartest person and you want to align yourself with the best and that the masses can not be expected to comprehend that which is genious, therefore you don't want to follow the cultural system your are brought up with and want to do like the Buddha.

He won't be able to refute the legitimacy of your strategy and at most will say he does not believe in Nibbana and the highest good but either way he will have to admit that you have a strategy and a worldview based on conviction that he can not disprove there and then.

If he was to develop conviction in that your strategy is the correct system of knowledge he would need to learn the Dhamma and come to agreement after reflecting on it.

OP: How can I explain to a non follower of the Dhamma that killing living beings intentionally has mental and physical consequences?

Here, I assume that the non-follower of the Dhamma might be persuaded by scientific research output and professional opinions in the field of psychology and medicine.

Robert T. Muller, a professor of psychology, wrote in the article "Death Becomes Us: The Psychological Trauma of Killing" (Feb 21, 2014):

Killing is often misrepresented in film as far easier than it is. In reality, the “duty” is mentally taxing, leaving most soldiers physically ill in the moment and often haunted by nightmares for a lifetime. Being responsible for ending the life of another human is a significant source of trauma ...

Apart from about two percent of individuals classified as “psychopaths,” who, because of deeply rooted personality flaws are unphased by the act of killing, most soldiers are unprepared for the task of ending the life of another human being. Many veterans report that ending even one life is enough to haunt them with painful memories and sometimes flashbacks.

Soldiers who have engaged in close combat are left with a much higher likelihood of developing post traumatic stress disorder then their counterparts who did not.

Stella Compton-Dickinson, a professional clinical therapist wrote in the article "The Psychology of Killing" (Aug 1, 2017):

My work with mentally disordered offenders who have killed, demonstrates how instinctual, mindless impulses result in high-risk behaviours. These become observable in body language and symbolic gestures, which can be explained and understood.

Dave Grossman, retired U.S. Army personnel and former West Point Psychology Professor, wrote together with Bruce K. Siddle in the article "Psychological Effects of Combat" (1999):

... To truly understand the nature of this resistance of killing we must first recognize that most participants in close combat are literally "frightened out of their wits." Once the bullets start flying, combatants stop thinking with the forebrain, which is the part of the brain which makes us human, and start thinking with the midbrain, or mammalian brain, which is the primitive part of the brain that is generally indistinguishable from that of an animal. ....

Among Vietnam veterans in the United States, PTSD has been strongly linked with greatly increased divorce rates, increased incidence of alcohol and drug abuse, and increased suicide rates.

​The above articles talk about the psychological effect of humans killing humans. What about the psychological effect of humans killing animals?

Ashitha Nagesh wrote in the article "The harrowing psychological toll of slaughterhouse work" (Dec 31, 2017), quoting those who have worked as butchers:

Former slaughterhouse workers who have since spoken out about their time in the industry have described feeling this change within them, and experiencing PTSD symptoms for years afterwards.

One hog sticker, Ed Van Winkle ... describing ... the dissociation slaughterers had to force upon themselves. ...

‘The worst thing, worse than the physical danger [of on-the-job accidents] is the emotional toll,’ Winkle said. ‘Pigs down on the kill floor have come up and nuzzled me like a puppy. Two minutes later I had to kill them – beat them to death with a pipe. I can’t care.’ ...

Another such man was Virgil Butler, who worked in a poultry plant in the US .....

‘The sheer amount of killing and blood can really get to you after a while,’ .... ‘Especially if you can’t just shut down all emotion and turn into a robot zombie of death. You feel like part of a big death machine. [You’re] pretty much treated that way as well. ...

.... ‘Many people who do this commit violent acts,’ he wrote. ‘They commit crimes. People who already are criminals tend to gravitate towards this job. You can’t have a strong conscience and kill living creatures night after night.

So, how does this relate to the Buddhist precept of not taking a life? The Buddha taught in SN 42.3:

When a warrior strives & exerts himself in battle, his mind is already seized, debased, & misdirected by the thought: 'May these beings be struck down or slaughtered or annihilated or destroyed. May they not exist.' If others then strike him down & slay him while he is thus striving & exerting himself in battle, then with the breakup of the body, after death, he is reborn in the hell called the realm of those slain in battle."

Also, from AN 10.92:

"When a person takes life, then with the taking of life as a requisite condition, he produces fear & animosity in the here & now, produces fear & animosity in future lives, experiences mental concomitants of pain & despair; but when he refrains from taking life, he neither produces fear & animosity in the here & now nor does he produce fear & animosity in future lives, nor does he experience mental concomitants of pain & despair: for one who refrains from taking life, that fear & animosity is thus stilled.

And what's the purpose of virtue (which includes the Five Precepts)?

From AN 10.1:

“Thus, Ānanda, (1)–(2) the purpose and benefit of wholesome virtuous behavior is non-regret; (3) the purpose and benefit of non-regret is joy; (4) the purpose and benefit of joy is rapture; (5) the purpose and benefit of rapture is tranquility; (6) the purpose and benefit of tranquility is pleasure; (7) the purpose and benefit of pleasure is concentration; (8) the purpose and benefit of concentration is the knowledge and vision of things as they really are; (9) the purpose and benefit of the knowledge and vision of things as they really are is disenchantment and dispassion; and (10) the purpose and benefit of disenchantment and dispassion is the knowledge and vision of liberation. Thus, Ānanda, wholesome virtuous behavior progressively leads to the foremost.”

As someone who has practiced Zen for 15 years and both hunts and raises most of his own meat, let me put in my two cents on the matter.

First of all, that person you were talking to is full of shit. But that's to be expected. Few modern people realize the deep bond that begins to form between the hunter and his quarry or a farmer and his flock. It is an interspecies intimacy that most will never know outside of a religious practice like Buddhism. By the time you find yourself aiming your arrow 20 yards from a deer, most of the time you know where she eats, where she drinks, where she sleeps, and who she hangs out with. You probably also know how old she is and how old some of her children are. She probably knows you too. The same goes for raising animals. When I kill chickens at nine weeks, I've raised them from the time they were one day out of the egg. I've fed them, cared for them, kept their shelter clean. Some will even demonstrate undeniably unique personalities. When I kill something, the experience is truly face to face. I'm not killing an animal, I'm killing a unique individual. And it hurts a little. That person you were talking to clearly never had a comparable experience. My guess is that most of the meat he/she has eaten has come in a little shrink wrapped package. They never felt the warmth of that animal's blood or felt the softness of its fur. If they had, they'd know full well why so many hunter-gatherer cultures have thanksgiving practices that they perform for the animals they've harvested.

So what does this mean in the context of Buddhism? Buddhism is all about developing intimacy. When we become truly intimate with the world around us, we don't have to raise animals to develop that kind of connection. We feel it viscerally because we directly experience our unbreakable connection to the world around us. We feel the bond that the farmer feels and what the hunter feels without needing to become hunters or farmers ourselves. This isn't mystical nonsense. The more we practice, the more these feelings arise...ultimately becoming just as obvious to us someone hitting us over the head with a sledgehammer. Ultimately, when we kill, it is 100% clear that we are killing ourselves. No one who feels that way can go through that action without experiencing mental or even physical duress.

How can I explain to a non follower of the dhamma that killing living beings intentionally has mental and physical consequences?

Kamma is intention. The intention dictates the mental and physical consequences.

This person started saying: "old hunter-gatherers, had no consequences nor problems with hunting".

Correct. They hunted for food; to feed their children; following the cycle of life; how nature created them.

If I tell them this is the Buddhist point of view they are going to label it as religious mystical nonsense.

The above appears to be your personal point of view rather than a Buddhist point of view. If the above idea was Buddhist, there would be specific quotes from scriptures to support this idea; where the Buddha instructed people to not kill for food. Obviously, in the Buddha's time, people would have asked him the same questions about how to gather food. Obviously, the later monks decided such questions did not warrant inclusion in the scriptures. We know the Buddha never taught vegetarianism because he allowed monks to eat meat from almsfood. The Buddha appeared concerned about killing with wanton violence (rather than about how people feed themselves).