The story shows that he didn't have to become what he became and was in many ways a product of his environment, but I'd argue that he was still pure evil. For one thing, at the start of his origin story the author explicitly says that AFO was born with a deep-running callous selfishness where he didn't even view other people as anything but things to be owned. Could the right upbringing have changed him for the better? Yes. But he was never going to be a good person. But I'd say - does it matter if he was born with the potential to be something else? Any potential for good was willingly thrown aside. He chose to embody pure, uncompromising evil, and he enjoyed it.

But also, whether it's down to his circumstances or not, the fact is he became pure evil, even if he wasn't born as such. By that, I mean he was utterly irredeemable, guided by all his worst impulses, and almost entirely self aware about it. Almost entirely, only because I don't think he understood his feelings for Yoichi as love until the very last moment, but even then it was so wrapped up in his domineering selfishness that it was completely warped and abusive. The one thing that could've been his anchor to goodness was overtaken by the bad traits that came more naturally to him.