The Nullifier by Erin M. Leaf

Contract killer Felix Zamaro isn’t looking for love—at his age, all he wants to do is retire. When he interrupts a robbery-in-progress, he stumbles into a mess: Nick Banner is in town to help his sister kick her loser boyfriend to the curb.
Nick never thought he’d meet a hot-as-hell assassin in the middle of a random coffee run, but watching Felix take down a couple of thieves in a crappy gas station has him questioning his rule against playing with straight guys.

When Felix’s next mark turns out to be none other than the boyfriend of Nick’s sister, all hell breaks loose. When Nick moves into the house next door to Felix, both men think fate is messing with them. However, it’s Nick’s unexpected career choice that really throws Felix off his game, and for a killer, that could be a fatal mistake.