Dark Hunter by AJ Adams

Rip Marston is a merciless killing machine. After a decade of hunting his prey, Rip joins the Zetas. The job offers protection as he practices his dark arts, but the Cartel are wary of the monster in their midst.
Finding a badly beaten unconscious girl, Rip sees an opportunity. Posing as her saviour will please the cartel – and provide him with his very own helpless captive.
Isabella Maria Franco is beautiful, wilful and used to making hard choices. Having grown up in the comfortable but lethal embrace of the Gulf cartel, she rejected a life of violence. But when a dark presence from her past returns, her world falls apart.
Betrayed and beaten, she escapes, only to find herself in a living nightmare. Surrounded by her enemies, one man stands between her and death. Terrified by her sadistic captor, she has little choice but to submit.
But embracing his darkness leads to consequences neither anticipated.