Catelyn's anguist at witnessing the death of what she thought her last living son, his wife and her grandchild to be caused her to take an innocent life before having her own throat slit, knowing all the while if she hadn't let the kingslayer go, the ending might be different.
Tyrion: Let me give you some advice bastard. Never forget what you are. The rest of the world will not. Wear it like armor, and it can never be used to hurt you. Jon: What the hell do you know about being a bastard? Tyrion: All dwarfs are bastards in their father's eyes.