In almost all cases I would say no. However I heard on the news the other day about a coach who was molesting the boys he was coaching. I thought to myself if someone had done that to one of my kids there wouldn't have been enough of them left for a trial.
I believe in doing what's right. Trying to stop a fight/rape is right, stopping someone from beating their child in public is right. Beating up drug dealers, child molesters, and just general *ssholes is not right. Violence doesn't fix violence 9 times out of ten.
Often - but only if it is NOT publicized.
Ye have heard that it hath been said, An eye for an eye, and a tooth for a tooth: But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.
Justice is impossible to attain within a lifetime. I condone defending yourself from bad people. Considering the number of bad people, I can ensure government employees will have a bad element that will try to hurt you. Just do the right thing and don't sweat it if the roaches assail you in droves and win. They are going to win anyway.
Not at all, best done when no one knows