I've red the Phillip Carlo book on Kuklinski and I enjoyed it as I did his other books on Gaspipe (Casso) and Tommy Karate Petoria, see my response on Mafia books forum.

I am however slightly sceptical about many of his confessions, in Phillip Carlo's later book about Tommy Petoria it mentions a murder which Kulinski supposedly was involved with, the murders of Carmine Galante and Leonardo Coppolla, others involved have denied any involvement by Kuklinski...

All that said he most certainly was as you say a Mafia contract assassin and most definately a maniac serial killer.

When they send for you, you go in alive, you come out dead, and it's your best friend that does it.