I don't feel I'm in a position to say who deserves what in society, and therefore I would never 'take the law into my own hands'. Also, I don't feel I'm in a position to say whether or not a girl who was raped or some guy who was killed deserved it because of their voluntary involvement with the people who committed the crime.
Mostly, I don't like the idea of vigilantism because I believe in karma; if everyone gets what's coming to them, why I should I try to punish them? Why not let fate take care of it?