Zamasu hated mortals. That much was certain.
He'd kept it to himself for so long while he was under Gowasu's tutelage; he'd wanted his master to believe he had put his negative feelings on the subject to rest, just to ease the old kai's mind. But, the truth of the matter was that it had only gotten worse over time. He hated mortals and their endless cycle of violence, of sin, of stupidity. He didn't understand why they sought to ruin their world with war and greed when the gods had so generously provided them with life and knowledge. It was baffling to him. He'd never understand it.
He'd often pondered if there was a way to fix everything—to end the mortals' cycle of foolishness once and for all. Maybe then he could do his job as a god and bring peace and beauty to the universe.
It wasn't until he was faced with his mentor's killer that he realized his dreams could become a reality.
Goku stood in front of him—or, at least, a man with Goku's face—a dark grin on his lips and a knowing look in his eyes. He held out a single green potara earring for Zamasu to take, seeming expectant.
Things happened quickly after that. He took the potara. He became Supreme Kai. He got acquainted with what was revealed to be himself from the past, who had apparently taken on the body of Goku to gain more power. He helped his past self kill his fellow kais. He began to enact his plan of divine cleansing upon the mortal worlds in each universe.
He felt alive. No longer did he mourn for the world as it fell victim to the mortals' foolishness. No longer did he have to sit in silent fury while he listened to Gowasu tell him how this was just the way things were, that this was their place.
He had a new place, now. It was as the bringer of judgement alongside his new partner, looking down upon all mortal life. He was a god, and he'd be damned if he didn't show them all.