Id assume most mobsters would think them exactly what they are; sick fucks. That being said, certain guys (Roy DeMeo, Tommy Pitera, I know they're the obvious ones but I know there's more)
could kinda sorta be considered serial killers, even though I dont believe they technically fit the mould.

Off on a bit of a tangent, but longtime Genovese capo Pete DeFeo was a relative of Butch DeFeo, perpertrator of the Amityville murders that spawned the franchise. Not a "serial killer" as such, (more a spree killer) but still an interesting tidbit.

Also, there's the unsolved case known as "The Axeman of New Orleans" in which a handful of people were murdered by being hacked to pieces with an axe in New Orleans in the early part of the last century. One theory holds that a low level local mobster known as John Mumfre was the unsub.

I for one have always had an interest in these sick fuckers, mostly the mentality and sheer insanity involved. Recommended reading has to be The Serial Killer Files by Harold Schecter; easily the best book on the subject Ive read to date. The guy breaks down basically every major case in the last 500 years.