I gotta go with Jace on this one. The Gemini Crew didn't leave loose ends. There were 3 reasons you would undergo the Gemini Method, as a matter of business, as a threat or potential threat, or murder for hire. There were some victims who probably didn't need to be whacked but posed potential threats as possible witnesses, if not eliminated, such as Padnick's son and Daniel Scutaro who was a friend of Chales Mongitore, a inter-mob, favor hit job.
When you look at typical serial killers such as Bluebeard, Ted Bundy, John Wayne Gacy, BTK, Charles Ng, they were driven by personal pleasure and mostly sexual gratification. DeMeo, Rosenberg and the Crew eliminated threats, bad business deals and fulfilled contracts. Occasionally, there were civilian casualties that couldn't be helped like the Ragucci kid. Cherie Golden was tragic, but she was no angel, and neck deep in John Quinn's auto theft operation, and knew too much about Roy. I think Sammy Gravano was scared of Roy at the time and wouldn't have called Roy a serial killer to his face, no matter what he says today.