Probably the Scatino bust out, purely because it was such a drawn out and malicious process as well as the fact he could clearly see what he was doing to Davey and Davey's family. Other than than that, the majority of the other ruthless acts he committed were of a some form of "street justice", I'm not trying to justify what he did, it just lessens the severity of the effects his actions caused. Ruining a civilian and his family's lives, not to mention an old friend, goes way above killing another murderer like Ralphie or the others.