Well, the story is that he USED to be VERY, VERY, VERY evil. So he made himself immortal(By removing his soul) so he'd never have to go to Baator/The Abyss, no matter how good he was in his later lives he couldn't atone for it.
I wish they'd make a PS:T 2 where his old partymembers go to Baator/The Abyss to fetch him back.... Then perhaps you could play some mercenary that they'd hired to aid them....
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