No Rest For The Wicked - Kresley Cole

Three hundred years ago, Sebastian Wroth's brothers forced him to become a vampire. Burdened with shame, he lurks in the shadows and yearns for death to free him from life's torment. Until he meets Kaderin The Cold Hearted, a beautiful valkyrie assassin intent on killing him, who ignites his will to live again. After Kaderin lost her two beloved sisters to a vampire attack long ago, an unknown benefactor extinguished all of her emotions in an attempt to save her from sorrow. She became known as Kaderin the Cold Hearted, a prolific vampire slayer. But when she encounters the handsome Sebastian Wroth, she is flooded with feelings again. For the first time, Kaderin is unable to complete a kill.