That makes everyone their own judge, jury and executioner.
Sorry but that is wrong on so many levels. You are basically saying that anyone can take justice into his own hands if he thinks the authorities aren't doing enough? that is an extremely slippery slope and not in accordance with the last of the Sheva Mitzvos Beney Noach.
I never said that he should take justice in his own hands. Hence why I keep referring to a deranged person, who doesn't care how law says to bring someone to justice. But if he does take justice into his own hands, ex-post-facto I would have no problem with it if is clear that the cop is guilty.
I can give a harsh example: Someone lived through Nazi, Germany. After the war, he murders the SS guard from his concentration camp. The Nazi was never tried under international law, and the holocaust victim would be guilty of murder. I believe, that when someone is willing to take justice into his own hands, most of us wouldn't mind the outcome in certain scenarios.