Assassin's Creed by Kurt Dysan

While the code of an assassin might not be lofty or idealistic, it sets standards for conduct. When he comes across an assault on the cruel and beautiful Lady Xena, he rescues her from would-be kidnappers. He finds her beautiful, grateful, and aroused by the spilling of blood. She encourages him to take her as a reward and he is happy to do so.
Pleased with his sexual prowess as much his ability on the battlefield, she wants him to stay with her and take her many more times. He is on another mission his creed dictates he must complete, but he offers safe passage to an inn where her soldiers await. Once there, he is able to live up to his creed, and complete his original mission, giving the haughty lady more than she expected.