Killer, by Dave Zeltserman

Aged hit man Leonard March is back in Boston after fourteen years in the slammer. And he’s lucky it was only fourteen-- turning state’s evidence on his former employer spared him a longer term.
But now, working as a janitor and living in a filthy dive, March is haunted by the memories of his murders and wracked with guilt over his non-existent relationship with his children. He’s playing out his final days, knowing that, sooner or later, his old boss’s thugs will come after him and make him pay for his betrayal. Either that, or one of his victim’s relatives.
There’s more to this sad old man than what meets the eye, however. While Leonard March may be repentant, just how much of the cold-blooded killer is left inside him? Before Killer is over, the reader will learn the chilling answer to that question.
This is Dave Zeltserman’s final book in the so-called “Man Out of Prison” trilogy, and fans of Pariah and Small Crimes might be surprised by the strangely contemplative nature of the story. On the surface, Killer is about regret, loneliness, and that old inevitable mortality thing. But scratch a little deeper and even darker themes emerge… it all leads to a truly shocking climax that could be Zeltserman’s darkest yet-- and this is a writer who has made a fine art out of dark and shocking climaxes.