These Murky Hells - Bloodlust, #1

Luna was trained to kill. She spent her entire life knowing that her purpose was to assassinate an enemy, eradicate a threat. But, once she does, something is unlocked within her. A thirst for blood, a hunger for death. As she learns more about her past and her new instincts, she realises she can't trust anyone. Not even herself. What happens when the chosen one chooses herself?