Luigi is a symptom of an overly inactive justice system, if you don't prosecute crimes then some people take the law into their own hands (as a matter of fact). Preventing vigilantism is why having a working justice system is important to a functioning society.

DPR is according to his defenders a symptom of an overly active justice system prosecuting crimes that shouldn't be prosecuted. Though I'm not sure I personally agree with that.

OJ was just a crook.