score:8
The background story to the first verse of the Dhammapada is of an arhat killing insects accidentally, because he's blind.
The verse says,
All mental phenomena have mind as their forerunner; they have mind as their chief; they are mind-made. If one speaks or acts with an evil mind, 'dukkha' follows him just as the wheel follows the hoofprint of the ox that draws the cart.
I think we're meant to infer that if you don't act with an evil mind (or act without an evil mind), that if the killing is accidental or unintentional, that's not as bad as intentional killing.
Something similar is found in the monastic rules (page 29):
In analyzing offenses for the purpose of determining penalties, the Vibhanga divides an action into five factors: the effort, the perception under which it is made, the intention motivating it, the object at which it is aimed, and the result. In some of the rules, all five factors play a role in determining what is and is not a full offense. In others, only two, three, or four play a role. For example, under the parajika rule forbidding murder, all five factors have to be present for a full offense: The object has to be a human being, the bhikkhu has to perceive him/her as a living being, he has to have murderous intent, he has to make an effort for the person to die, and the person has to die.
If any of these factors is missing, the penalty changes. For instance, object: If the bhikkhu kills a dog, the penalty is a pacittiya (i.e. a less-severe penalty]. Perception: If he cremates a friend, thinking that the friend is dead, then even if the friend is actually alive but severely comatose, the bhikkhu incurs no penalty. Intention: If he accidentally drops a rock on a person standing below him, he incurs no penalty even if the person dies. Effort: If he sees a person fall into the river but makes no effort to save the person, he incurs no penalty even if the person drowns. Result: If he tries to kill a person, but only succeeds in injuring him, he incurs a thullaccaya [which is another less-severe penalty].
Upvote:1
To add to the already good spread of answers here, carelessness is something to be avoided. Stepping on an ant can still add up, especially if throughout the week it becomes ants.
If you can avoid it, but do not avoid it, you are in some sense complacent.
Now if there is the situation where one was completely unaware of the situation and did not see the bug until it was too late... Well that is different.
But if the sidewalk is flowing with ants, and one knows this, one should watch their step as they traverse the path... And maybe put ones phone away for the few minutes of travel, and try to just be, and observe things as they are.
Upvote:2
Sinful Killing/Intentional Killing
You can determine yourself whether it is intentional killing or not by checking as follows.
Any of three is happened on your mind? You committed "Killing" karma. It is subtle point that you break the first precept or not by completing three steps above. But it is bad Karma if you did any one of the three steps and you should expect reflection of your deed. Sometimes the state of mind and transition of mind during killing process was very fast and one or all of the steps seems not happened. But careful analysis might show it happened very fast in your mind. The most important point here is let bygone be bygone. Do not let the notion that you committed killing linger in your mind. You need to move on. You need to determine yourself you will not break first precept in future. Every end of the day you didn't do killing you should have notion and keep that notion with pleased yourself to multiply good karma.
Upvote:6
While usually we understand an "action" to be equated to its effects (e.g. the act of killing would be the act of "turning" a living thing dead), in Buddhism, intention is often equated with action, as per the sutra AN 6.63:
cetanāhaṃ bhikkave kammaṃ vadāmi
Intention, I tell you, is kamma (action)
Thus, wholesome and unwholesome actions are not simply judged by it's effects. They are also not judged solely by one's intention, but generally also by one's (lack of) wisdom (i.e. by one's ignorance of the workings of dukkha).
In Buddhism, it's not a dilemma a well-intended person doing something foolish (both the intention and the foolish act are distinguished).
Nevertheless, I think one is ultimately not really accounted for what lies outside the touch of the senses and the consciousness of the sensed data (as exemplified in @ChrisW's answer). Thus, killing a living being by stepping on it without having sensed it was there is not quite considered an act of killing.