I think it's a great mystery, and I like it just the way it is: Mysterious.
He says in the game that he doesn't know what he did. That's interesting, as it implies that he didn't really do anything blatantly wrong. Plus his conduct in the Land of the Dead shows that he's a good guy, he's just seems sad, yet accepting, that he's stuck paying off a debt.
But it's not only that I like the mystery that I don't want to guess why, it's also that it's ridiculous to even attempt to theorize on.
Think about it. What would we be guessing? His entire life? What do we have to go on? He's in moral debt, he doesn't even know why, and it would have been different if he had had a car like the bone wagon when he was alive.
The possibilities are endless! And there is no way to validate anything.
God is a placebo with nasty side effects.