Snakes Don't Walk — Marsell Morris

He discovered that he enjoyed killing, after he, as a young man, murdered his ex-girlfriend because she spread lies about he being gay. Up until that turning point in his life, he could never find happiness.

It was a long time before he had a chance to kill again. And when he did, he knew he would do nothing else. But how to make a living taking peoples lives?

His name was Kelvin. He called himself K, and he was an emerging psychopath. Could there be a worse combination -- a hit man, who was also a psychopath?
His desire to kill for money happened by accident, when he met a distraught older man in a dark dingy, smoke filled, dive, who was trying to drown his matrimonial problems. K leaped on the opportunity and convinced the beleaguered, older, man to let him take the cheating wife's life.

He soon discovered that he loved killing so much that he became sexually aroused while only thinking about the next hit -- for cash or kicks, it didn't matter, he had to kill. He eventually combines sex and murder, and the monster is loose. After murdering the wrong person, his destiny is sealed.
His death is as violent as his twisted life.