"I would like you to agree with me that using you to destroy the Enemy is worth it. I will do it anyway, because it is worth it, but I think you'll be happier and more productive and we'll be likelier to succeed if you agree with me. And right now the only thing you know about the Enemy is that he tortured me and you'd be warranted in thinking I probably deserved it."
"Well," she says magnanimously, "I don't know what you were like before that."
"I'm immortal. Like, really immortal," she reminds him. "And if you were going to be handing out promises not to give me any orders that would probably have bumped you up in trustworthiness to make you a good contingency master so I'd have wanted it a little narrower; rescindments are orders."
"Noted. But you did not meet me five hundred years ago when I remembered more than scattered chunks of my life and the Enemy could not talk in my head at will and I was not in constant pain and I had people I loved; you met me now. And now if I can I will use you to end him, and then I will find you someone you can trust and have such a good mutual agreement with, and then I will figure out how to stop existing, which has been the only thing I've wanted for a very long time."
"If there's a gate before you do that I could fix your hand and whatever else is hurting you."
"...by making sure you have a person here to your liking who can feed you, and a gate back if the means for any such thing exists, and rescinding all my orders. And I can leave, if you like, or pretend if you like that I'm off on a grand adventure to discover myself, so you don't feel any sense of responsibility..."
"I'm not very likely to believe you're on a grand adventure to discover yourself."
"I wouldn't try to stop you from killing yourself if that was what you wanted to do but I'm skeptical you can avoid it negatively affecting me. I wouldn't kill Thorn if he were mortal and I had the flexibility - I'd have him turned him into a sparrow, even, not a snail, I'd leave him his consort, I think she actually loves him -"
"Well, I'm pretty sure Thorn would rather be a sparrow. Like I said, I wouldn't stop you."