How concerned should Buddhists be with the actions of others?

Isn't there a rule, about eating meat, something like,

• Don't, if the animal would be killed specifically for you (e.g. because you asked for or paid for it)
• OK otherwise (if you're begging for any type of food, and the donor chooses to give you meat)

A moral (i.e. a general lesson) that I read into that rule is,

• Avoid being the cause of suffering; for example, don't encourage (pay money to) a butcher.
• Optimize the situation which exists in the present; if the creature is already dead (through no fault of your own) and meat would benefit your body, then why not (waste not, want not)?

But I don't know. If for example that "friend or nearby person" were a child of yours, I think you should probably instruct them, to improve behaviour in the future: "Don't steal; I'd prefer you didn't kill; ...".

Trying to instruct an adult friend in way that might, I don't know, have negative consequences; so if you were to try to do it you should do so "skillfully".

One practice you might try in such a situation is use it as motivation to contemplate the unsatisfactoriness of samsara and resolve to develop true renunciation.

No matter what we do, as long as we are samsaric beings we are contributing to the suffering of others. Our very existence acts as a cause or condition for the suffering of others. The food we eat, the water we drink, the shelter we take... the basic factors we need to live all act as a cause or condition for others suffering.

This comment to your question was very poignant for me:

I'm glad you asked this question as this came up in my life recently. I was marveling at a huge group of ants gathered on my brother's porch. This inspired him to grab a can of Raid and spray them all to death.

This person had no ill will towards those ants nor took any non-virtuous action with regard to them and yet acted as the cause or condition for the death of those ants at his brother's hand. This is the predicament of samsara and I think closely related to the Sankhara-dukkha or the general unsatisfactoriness of this conditioned existence.

Contemplating like that might accrue to the cause of generating true renunciation for samsara.

@Robin111, this is a great question and a turning point from "Hinayana"-type-mentality to "Mahayana"-type-mentality (note: I'm not speaking about the schools Theravada and Mahayana, as both of these two types of mentality can be found within both Theravada and Mahayana schools).

The practice of transcending one's ego involves continuously reflecting upon one's actions and choices to see if they are biased with ego perspective. As this practice is refined, you may find yourself finding egoistic motives in seemingly egoless choices and vice versa. This is normal and good. Keep going until the end ;)

There is no benefit to accrue to anyone except personal actions that lead to the ending of suffering.

Someone killing a pest doesn't "benefit" you, it gives you some short-term sensual pleasure--the pleasure of being outside and feeling a cool breeze without a determined gnat landing on your face. Or the pleasure of eating food/drink without having to pay for it. But these are certainly not permanent pleasures, nor ultimately fulfilling.

Instead of worrying about who did what action and what pleasure resulted, try thinking about why you think superficial pleasures like these are beneficial? So long as a mind thinks these are good things worth pursuing, it will pursue them.

And for the comment underneath you, also about killing insects:

I felt awful because I'd inadvertently caused hundreds of deaths

Something to think about: if you can't accept the death of an ant, how will you deal with your own death, when it comes? Think about the hatred or egotism that causes a person to kill ants and how it should be abandoned, don't think about how awful death is and how it should be avoided. Nothing anywhere is avoiding death and it's happening on a much grander scale in nature than it is with cans of Raid.

One aspect of Karma is that it is contagious nature and cumulative in nature. Say you have bad Karma; when the fruit ripens, in many cases some being near you should harm you and be instrumental in the karma giving its fruit. Then the being who is instrumental accumulated bad Karma. In the future some one else (in most cases) must be instrumental in giving a means for the karma to manifest.

Also due to the liking (or the lack of it) most people tend to be born together in Samsara with a higher degree of probability.

Hence if people around you do negative karma, then there is a possibility that there will be a negative vibe with all associated people due to the complex interactions.

Hence it is best to encourage your circle to be as positive as then can and avoid negativities but the best extent you can. This give two benefits:

1. Good Karma for you for doing a good deed
2. Collective good Karma (or lack of bad Karma) means you would accumulate more positivity through the process

Also being collectively good or doing good deeds as a group has better results than just doing as an individual.