"Serial killers are a bore in my book" (M. E. Smith)

Professional killers are much more interesting, imho. Take for example Roy DeMeo and Sammy Gravano, who both killed for the Gambino family on a regular basis at about the same time (there is an interesting Youtube video where Gravano recalls his run-ins with DeMeo, whom he disliked very much). Or take a political killer like Nikolay Yezhov, who organized the Stalinist terror in the mid-1930s -- until the Boss decided that it was enough and Yezhov himself was executed (along with many of his henchmen); Lavrentiy Beriya tried to make sure that he never met the same fate. In the end he did - after he had buried Stalin and ended the anti-semitic campaign of Stalin's final years (the only good deed in his life).

It should be noted that Yezhov's adoptive daughter recalled the two years she spent as a part of his family as the only happy time of her life. And according to his son's memoir, Roy DeMeo was not only a ruthless killer, but also a loving father.