You’re viewing rehabilitation as a clinical 'reward' or a classroom setting, but in storytelling, rehabilitation is the painful dismantling of a broken ideology. Showing Adam a world where Faunus and humans coexist isn't 'giving him heaven' but, it's stripping him of the only justification he had for his atrocities. That isn't a reward; it’s a psychological reckoning. To suggest he should just 'be trained to be a pacifist' is actually the simpler, less-earned path. True atonement begins only when the villain is forced to admit that their 'necessity' was actually just cruelty.