Lady Death by Beth D. Carter

An act of savage violence shatters Keres’s life. Her heart and soul demands vengeance. To hunt a monster, she must become a monster.

Ronin is tracking the person who’s been sniping his marks, and is surprised when he finds the beautiful red-head wielding the deadly knife.
Sparks fly between them. Not of love, but of anger. They don’t trust one another, yet can’t seem to deny their attraction.
When the Death Riders are attacked, her sacrifice is demanded. The unsteady foundation she’d been balancing upon crumbles, leaving her to face alone the nightmare that created her.