Sometimes, in his weaker moments when he was alone with his thoughts and allowed himself to ponder such things, Zamasu wondered if taking this body was really the right decision.
The thought had first occured to him after his first encounter with Trunks, when he'd felt the pull of battle taking over his mind. He'd never felt such a thing before as a kai; such things were below him, or so he'd thought. He'd never lost his head during a sparring match. He'd felt the urge to purge the world of the unsightly mortals that stained it, sure, but he'd never felt such a strong bloodlust as he did when he fought in this body. He didn't notice it at first, but when everything was done and they'd both gone their separate ways, he had a moment to think about what he had felt, and it was jarring. He had to collect himself and regain his composure before he completely lost himself in the adrenaline that came with the thrill of the battle. That was when he'd first wondered if giving up his divine body was the right thing to do. He'd wondered, what if he lost himself? What if he lost sight of his end goals and ideals? What if this body was having too strong of an effect on him? What would he do then?
These questions continued to plague him as time went on. For a while, he was unsure of this body and the effects it would have on his psyche, and it worried him, though he made a point not to show it. It wasn't until he stopped to think about his future self that he began to see the answers.
His twin was immortal. That was his solution to the problem of weakness in the face of his enemies and the trials they would go through in order to execute the Zero Mortal Plan. With an immortal body, a body truly fit for a divine being such as himself, nothing could stop him. Nothing could strike him down and keep him from his goal of cleansing the universe. Zamasu, on the other hand—or, rather, Black, a name he'd grown oddly fond of since being dubbed as such—had found a different solution. He'd taken the power and potential of a saiyan for himself, and he'd made it his own. He'd learned how to use it, how to harness it, how to execute it for his own purposes. He'd achieved a new form that no saiyan had ever dreamed of achieving, and he'd made it an indication of his status and progress. He'd taken something ugly and made it beautiful, fitting for a god such as himself.
Still, the question remained: Had he made the right decision commandeering this vessel?
The answer became clear to him as he observed the rip in space that he'd created with his newly-formed energy scythe, a weapon truly befitting a crusader in a quest such as this. He had become one with this body, yet he was not completely in control of it. He had accepted this, and he was embracing it, now. He enjoyed the uncertainty, the feeling of not knowing what new surprises this body had in store for him. He welcomed it. It only proved that he had made the right decision, and that he had obtained something with exponential potential. He had the ability to continue pushing through his boundaries in ways that he couldn't have imagined in his old body. He would even go so far as to say he was glad that he'd discarded his old flesh and taken on this new, more flexible form.
He may have not had total control over his body, but he'd decided that this might not have been such a bad thing after all.