Have you ever heard the saying "Two wrongs don't make a right"? It's fairly popular as far as I know. The thing is, crime is very clearly defined as something that is against the law, but pretty much nothing else in your question is very well defined at all. A major reason vigilanteism is illegal is because people are idiots who often care more about making themselves feel better by getting revenge on someone they believe to be unjust than actually serving justice. It doesn't matter if the person is actually guilty as long as the punisher believes they are. That is why due process is so important.