i know this is probably a hot take among jjba fans, but i wouldn't consider dio a literal ‘pure evil’ villain, as i don’t think he’s completely devoid of redeeming features — these being his traumatic childhood, and his very limited capacity to care about some people, namely his mother and pucci.
i would, however, put him in the ‘nearly pure evil’ category alongside yoshikage kira. both are socio/psychopaths who were based off irl serial killers/mass murderers, and despite being abused by their parents, they were already born with some psychopathic tendencies that'd later be exacerbated by their traumatic experiences.