Assassin Anonymous by Rob Hart

In this clever, surprising, page-turner, the world’s most lethal assassin gives up the violent life only to find himself under siege by mysterious assailants. It’s a kill-or-be-killed situation, but the first option is off the table. What’s a reformed hit man to do?

Mark was the most dangerous killer-for-hire in the world. But after learning the hard way that his life’s work made him more monster than man, he left all of that behind, and joined a twelve-step group for reformed killers.
When Mark is viciously attacked by an unknown assailant, he is forced on the run. From New York to Singapore to London, he chases after clues while dodging attacks and trying to solve the puzzle of who’s after him. All without killing anyone. Or getting killed himself. For an assassin, Mark learns, nonviolence is a real hassle.