Under the Bones - Kory M. Shrum

Enemies make the best allies. 
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Louie Thorne is good at only one thing. Killing. She knows peace only when she kills and this relentless thirst drives her to hunt the world's worst crime lords. But when her cancer-stricken aunt makes a dying wish, Louie is torn between her own dark nature and a promise made to the only family she has left. 

Enter Konstantine, bastard son of the crime lord who killed Louie's family, and now the leader of the Ravengers—a ruthless crime faction from Florence's underworld. He didn't rise to power without making enemies and now one of those enemies has made Konstantine the most hunted man alive. Konstantine's only hope of reclaiming his throne is to trust the woman who wants him dead...