(He really doesn't like being the powerless one. He really doesn't like having to talk about his personal sequence of traumas. But whatever. This is probably at least more likely to leave his brain intact than throwing his lot in with Orochimaru.)
"I was a shinobi of the Hidden Leaf, the main organization of chakra-users in the Land of Fire. I'm one of two or three people with my bloodline, and was the only one still in the Leaf. The other two are stronger than Orochimaru. Orochimaru decided he wanted my bloodline. He found me, put a seal on me that fucked with my head, then ditched me to live or die on my own strength. I lived. He later sent his own shinobi to retrieve me. I told them to fuck off. They were stronger than me, and could manipulate my seal, so got me angry enough to trap. Next thing I was really aware of was being in one of his lairs. I've been here two years, mostly trying to earn his trust and sabotage him."
"I'd like people to stop hurting each other. My earlier plans were pretty vague, mostly involved killing Orochimaru and one other asshole. Figured I'd probably die somewhere in there, and if I didn't I'd have enough of a reputation to start un-fucking Orochimaru's research without being bothered too much. This world sucks, but - Orochimaru has a lot that could do good. With 'creating objects' available as an option - I don't know if our political problems can be fixed with ending scarcity and putting back the dead, but there's at least been some respected politicians who weren't dickheads."