This was a mistake. It had to have been.
Black had confessed that he had changed his mind about their mission to cleanse the world of its mortal infestation. Zamasu hadn't taken kindly to this idea.
And now, they were here, with Black being hurled to the ground and skidding through the debris of the wasted city until coming to a stop by means of crashing into a nearby building. He didn't want this. Neither of them wanted this. But, this is how things had to end between them.
Zamasu was just as pained as Black was, truthfully, even if it wasn't obvious. He'd trusted Black. He'd followed him to the ends of the universe and back. He'd spent all his time with him. He'd go as far as saying he considered him his closest friend. But he supposed things changed. Now, as he hovered in the air and charged up an energy blast to fire at his former companion, he had a look that only gave away the anger he felt at the situation.
Black removed himself from the rubble of the building he'd crashed into, stepping out into the open. He looked up at the sky where Zamasu was floating, a heavy frown on his lips. He didn't want to fight him. He wanted to just leave back to his home timeline and allow the other to continue his work here; that was his choice to make. It was not Black's timeline. He only wished that the other could share his realization so that maybe he could help him fix everything to the way things were before all this happened.
At some point, Trunks had appeared on the scene. It must have been due to the ruckus the two gods were causing with their scuffle. He looked from Black to Zamasu and back, realizing quickly that the two were fighting each other, and he wondered what was going on.
Black ignored him. He was no longer concerned with attacking the boy; after all, he had been one of the reasons he'd come to his conclusion in the first place. Instead, he launched himself at Zamasu with his energy sword drawn.
Zamasu fired off a barrage of energy balls, which the other quickly and gracefully sliced through. Black closed in, and the two began fighting with their swords, engaging in a deadly dance with what seemed like equal footing.
Black knew he couldn't defeat Zamasu. What good were his saiyan powers up against an immortal god? He knew better than that. He was simply trying to bide his time until he could find an opening to go back to his own time and leave this world and his former partner behind.
He hated that he had to do this. He hated that he'd led the other down this path only to turn on him at the last second and leave him there, confused and angry. He'd not only caused trouble in his own timeline, but the future as well. He knew he had a lot to atone for. He had accepted this much.
This was indeed a mistake. Black had made the mistake of turning on his teacher and wreaking havoc in the future world. Zamasu had made the mistake of listening to his past self and then trusting him to stay with him to the end. They'd both made their own mistakes.
It was only a matter of time before one of their quests came to an end. Both were determined to make sure that it was not their own.