Make him start out as a tool for the cult, i.e. they abuse his loyalty as much as his father abused him. He'll take it, but pass it on to his victims by truly ripping them apart, until he realizes that to be content he needs to be in control of his life, so he turns on his allies and friends, kills them all (or at least most of them) and takes control of the cult, where he orders every member to kill off their family to prove their loyalty, and after that, he kills them, and strikes of on his own, killing all who try to manipulate him.