The way I learned is that you try not to meddle unless you have wisdom (meaning you have some kind of enlightenment). My opinion is that unless I was the one tormenting other beings, I wouldn't meddle in other person's affairs, specially by killing. This is a good example on the importance of finding a teacher, a Good Knowing Advisor, someone that can dispel these doubts for you.

There's the obvious self-defense exceptions, if a lion or a person would attack me or my family I would shoot him, but an enlightened being may act differently because they may understand something we don't.