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Oberon guy here. I was GMing a long-ass running campaign with a group of friends, five to be precise. Two of these guys I know are lore buffs, they love learning what they can about every facet, while keeping things in-game and not metagaming. All five are really good about that.
Anyhow, there was a boss, a recurring villain if you will, who was questing for seven powerful artifacts, each based after one of the seven sins. Long backstory short, she was the last of a dead race whom these artifacts belong to, and the main BBEG (Whom the players did not know about for 70% of the campaign) had altered her memories. She used to be lawful good, but was now Neutral Evil due to these false memories that basically stole who she was. The party encounters her many times, always barely winning due to her being quite strong.
Final battle against her comes along. At this point, two of my characters have learned a significant amount of her backstory. She was the equivalent of a saint before her defeat and the false memories. After a nearly five-hour boss fight (Lots of planning, tactics, and action) everyone was still alive but absurdly low on HP. It then came to a deciding factor.
Kill her, or allow her to live?
She was dying from her wounds, all her artifacts having failed. The two that had learned knew that her actions were not entirely of her own fault, she was being controlled. The other three were all 'Fuck that, she's a bitch.' Nearly two hours of in game Roleplaying, out of game debating, and finally, they decided to allow her to live, since her original memories returned and she was on the brink of going crazy with two sets of memories, ones that were not true and ones that were. They helped her overcome it. She now runs an orphanage in the main town and provides the party free lodgings, and helps out the city. Even those who wanted to kill her admit it was the most fun post-boss event they've ever done. Now I am a GM for life for that group.