Rogue by Elle James

Rourke “Rogue” Logan has hunted killers across the globe, but none as slippery as the shadow assassin known only by the call sign “Onyx.” Rumored to be operating inside a powerful Texas organization and linked to a recent high-profile killing, Onyx has become a ghost the government desperately wants erased.

When intel comes in from a trusted former agency contact, Royce assigns Rogue a single mission: track down Onyx and eliminate the threat—clean and quiet.
But the closer Rogue gets, the more the pieces don’t fit. Onyx leaves no signature, no motive, no trail that matches an assassin for hire. And when Rogue finally corners the target in a remote corner of the Texas Hill Country, he discovers the truth that blows the mission apart—

Onyx isn’t the ruthless killer he was sent to destroy. Onyx is a woman on the run…running from the real monster.
Now Rogue must decide who the true enemy is—and why someone with power is desperate to see Onyx dead