Down on Your Knees by Lee Thomas

Denny "The Bull" Doyle steps out of prison only to find a low-level gangster attempting to take over his organization. Brendan Newton is a newbie to the gang who's spent too much time in front of the television, building a grand fantasy about the machinations of the underworld. He's naive and weak, but he may be the only chance Doyle has. The Bull's associates are being murdered in violent and bizarre ways, and the next target is his beloved, though wholly sociopathic, brother, Jordie. Behind it all is Malcolm Lynch, a sadistic gangster who has more than guns and knives at his disposal. He's a sorcerer intent on building an empire, and Doyle is the only thing standing in his way.

"Down On Your Knees, then, really strips the horror/crime genre down to its bare essentials and gives its readers the down-and-dirty details. And, make no mistake, the final few chapters will leave you breathless in your ringside seats, still clutching your half-eaten popcorn as the house lights come up and you think, “What the hell?” Yes. It’s that good." - Out in Print