Roy was the real psycho, Anthony, Joey and Chris were just his minions who saw Roy as some sort of father figure and did whatever he wanted them to do. They killed people in such way because Roy told them to and gave them confidence that he knows what he's doing and that these murders won't be noticed by the LE. I think Roy deliberately recruited young punks so that he could train them into anything he wanted and create his personal little army of 'psychos'. Without Roy they would be just some random street punks. After Roy died they didn't do anything special. If they were really serial killers they would probably kill random people in jail and we would here about them.

IMO Roy is the only really 'fascinating' person from that crew, it was basically him who "created" the twins and Chris - he recruited and schooled them, they were his "project". He started all those rackets the crew was involved in and made a ton of money. It was his decision that the crew would operate in this way and others blindly followed him.