Nothing Good by R. J. Piper

Charming and irreverent hitman Rainy happily holds the title of deadliest man in Miami. Or, at least, he used to. Things have started to go sour now that a new crime lord has moved to town, bringing along his right-hand man—the dour, dry-witted, totally fun-hating ex-military assassin known as Adler. When the two are contracted to kill each other, it becomes clear that Rainy may have finally met his match. Adler is clever, infuriatingly competent, and stupidly attractive. Oh, well. Rainy’s employers hired him to kill Adler, but they never said he couldn’t sleep with the guy in the meantime.
As their cat-and-mouse game grows deadly, though, Rainy starts to suspect that he may have gotten in over his head. He and Adler have more in common than he expected—both the painful pasts they’ve been running from and the truths about themselves they don’t want to face. Maybe Rainy doesn’t want to kill Adler—in fact, he might even love him.

And that’s definitely going to get him killed.