The Demeo crew murdered between 75 and 200 people (FBI Estimate).
Even Gotti gave Demeo a wide berth.
Casso went paranoid crazy when he was co-boss of the Lucchesse which is when he started clipping his own men left right and center - where as Demeo was only ever a Gambino soldier.
As far as mob killers go, they dont come much worse than Demeo
Maybe "Tommy Karate" Pitera (Bonnano) is almost on a par.

For sure, read "Murder Machine" and "For the sins of my father" (by Roys son Albert).
Both books are excellent.
Last I heard, Albert was trying to get his book made into a movie - dunno if that will ever happen though?
But for me, Roy DeMeo takes some beating in the contention for the title of the mobs worst killer