It could be argued that one of the main purposes - if not the primary purpose - of the legal system is to prevent vigilantism.
One of the most essential elements of any Batman story - more so than Bruce Wayne's money or his parents' death - is that the police must be shown to be either incompetent or corrupt or both. For whatever reason, when the Joker or Mr. Freeze or the Riddler get up to their old tricks, the police are insufficient to handle things.
In much the same way that poverty excuses and therefore promotes petty crime - who could fault a man for stealing bread to feed his starving family? - an incompetent or corrupt or prejudiced or useless legal system will excuse and promote citizens taking the law into their own hands.
And then, of course, one has unintended consequences and misunderstandings and retribution and that whole "cycle of perpetual violence" deal you may have read about. It's a human species thing.
Anyway. I hope the whole tortured metaphor of Jamie and Andre and Roscoe all being "related" isn't too heavyhanded. Let's move things along, shall we?