I doubt it is true but a Harry Bosch novel has a scene where 4 convicted serial killers are playing cards and it is pointed out that observing the scene one would not immediately realize that between them they had killed more people than the cards on the table.
I remember reading that book. Some truth as there are gang bangers and such who have killed scores but no proof. Four serial killers playing cards is a BIT over the top as they’re usually in lock down for the duration but I haven’t seen everything.