Final Day by Megan Erickson

When his boyfriend is murdered, Erick Lee makes it his life’s mission to hunt down the killer. The question is, how far is he willing to go to catch him?
Erick has been asked to team up by a dangerously captivating hitman who has reasons of his own to want this man dead. But can he trust another killer? And do two wrongs ever make a right?
Everett “Tarr” Hawk isn’t just some lethal hitman. He has his own code of honor. With Erick and Tarr facing a common enemy, it’s only by combining forces that they’ll survive. But Tarr’s code doesn’t include a fierce attraction for someone he’s sworn to defend with his life. Because in Tarr’s world, every night presents a new danger . . . and listening to his own heart could get them both killed.