I don't even like getting into scuffles with people. There's only one scenario I make exception to. Somebody assaults or grievously harms family or friends of mine then I have no restrictions. If family or friends were killed, then there'd be no place they could go that I wouldn't search for.
To be honest with you, I was considering it in october when my friend was killed. He was killed by a drunk driver. Not intentionally, I know. But I wasn't far off from checking out of the hospital and finding the driver's name.
I stopped thinking about it once I realized that the driver would likely live with that for the rest of his life. It would be more pain than anything I could do to him. And killing him would just get him a free ticket out of dealing with the consequences of his actions.