Yep. Agronak gro-Balog, the Arena's Grand Champion in 'Oblivion.' He was always at least polite while the other gladiators (besides the unnamed Blue Team gladiator) were all assholes up until the point that I was poised for success.
The problem, however, was twofold: I had actually accepted a quest from Agronak to find evidence of his heritage. His nickname was 'The Gray Prince' because of his alleged aristocratic bloodline, but he had no proof of it. So I ventured to a dark, forsaken tower and discovered...his father. A vampire. Agronak's mother had actually sealed the father in his own keep when she discovered his secret, and he'd gone mad from lack of blood in the meantime.
So I returned to Agronak with the news and proof and...he was heartbroken. Like, really. The friendly, talkative gladiator was gone, replaced by a black hole of utter misery. Then when I got out into the Arena for the title fight, he drew his sword, then stood still and told me to get it over with quickly.
Bear in mind, the Arena fights are all to the death, so he was essentially committing suicide by gladiator. True to his word, he didn't take a single swing, and I was still inexplicably heralded by the announcer as having rightfully claimed the title of Grand Champion.
Then I got back into the gladiators' preparation area. The quartermaster and overseer were, of course, ecstatic that I'd won, even if they had been massive pricks for the better part of my stay there. But then I spoke with the unnamed gladiator I mentioned at the start of this post. He'd been easily the most friendly person there. He was eager to share information if I asked for it, and he always wished me luck on my way into matches and congratulated me on my way out.
He stared at me with shock on his face and said (paraphrased), "You...you killed Agronak? He was one of my oldest friends. Well, Grand Champion, I hope it was worth it." The transition from shock to utter scorn was stunning to me at the time, and I felt like the worst person on Earth for what I'd done. It was the exact opposite of how the story was supposed to go: I'd murdered one of the few people who'd treated me well from the start and alienated the other, and all for the approval of assholes.
...and it didn't help that when I fell asleep on one of the Arena bedrolls, a representative of the Dark Brotherhood showed up to congratulate me on my murdering prowess.