How would you go about diagnosing and treating Dexter?

Hopefully people responding have seen the show Dexter and have some idea of his backstory and psychology arising from witnessing the traumatic death of his mother at an early age. In the show, Dexter is supposedly a psychopath, and in later seasons his therapist attempts a number of very questionable methodologies to get Dexter to find his inner humanity. But I'm really curious if the diagnosis is even correct and what the best way to 'treat' somebody like Dexter would be?