Vigilanteism. Those cops were taking the law into their own hands and not worrying about the consequences. Yes, Harry almost does the same, but like he said, "Until someone comes up with a better idea, I'll stick to the system." (Something along those lines).
These guys just went around and cold bloodingly murdered these people. Yes, those people did need to be served justice, but that kind of justice? Like that? No.
And these guys doing it that way, doesn't help the relations between the public and the police force. There's already people out there who don't trust or like cops, and then this scandle comes along, what would the public think?
Everytime they get pulled over they're going to get blown away.