If it were in self-defense, heck yeah. But otherwise? No. If the person had killed my family/friend(s), but gotten caught by the police, I would leave them to either get the death sentence (which I'm opposed to) or life in jail. 'Cause... killing doesn't really bring my family back, ya know? =/

If they hadn't been caught by the police, and were still out and about, I might. In a way of making sure they don't show up in my house randomly in the middle of the night to finish the job, or kill anybody else.