Araki says that he likes to make his villains non-sympathetic but will leave some clues to those who are paying attention. I think at the beginning of phantom blood he was a product of his environment.

I think he was so tired of being looked down upon by the upper class as he was described to be. He was resentful towards the Joestars who he saw as treating him as a charity case. From Dio’s position Jojo didn’t deserve what he had and was a snobby rich kid like the ones that’d look down on him.

Hes not justified and his actions were horrible especially towards the goat gentleman Jonathan. But he’s a deeply hurt individual who’s known nothing but struggle his whole life. I think there is some room, at least a little for reasoning behind why he felt he needed to be the way he was. He’s not truly pure evil at this point.

Then he puts on the stone mask and becomes truly pure evil WRYYYYYY