The Once and Future King Book 3, Chapter 8
Lancelot helps a woman get her lost falcon out of a tree, but it turns out that she only asked for his help to trap him-her husband wanted to have a chance to kill him without his armor on. Shocked, Lancelot nevertheless defeats the knight and killed him. He even feels bad about it. When he comes upon a married couple fighting, and tries to end the fight, the husband tricks Lancelot into looking away and then cuts his wife's head off. Even though Lancelot is furious at the man, he cannot kill him--even the fact that he wants to makes him sick. All the people he beats in fights, he sends back to Camelot to kneel before Guenever.