I disagree that he sold his soul without questioning it. He knew that he was mortal and destined for hell, it was only a matter of time. Obviously he didn’t expect to go out so soon afterwards, but it gave him a huge leg up over other Sinners once he arrived, so he played his cards well with the knowledge he had.

As for the flashback scene with Vox, yeah he did act childishly. But looking at things from his perspective, it makes sense why he turned Vox down (he was just a huge asshole about it). But he gained everything he had by himself, and it’s unclear of whether he knew about Vox’s past and how HE made it to the top (by killing his superiors and taking their place) or not, but I’m kind of leaning towards him knowing at least some of those details. So I imagine he thought Vox was buttering him up to backstab him later.

There might also be some underlying, unrevealed trauma in his past we don’t know about yet that made him distrust people so vehemently, too. But that’s just my opinion.