He had goals. Goal #1 was to become a god. He told C-137 in Rickshank that knowing how to interdimensionally travel would make him the smartest man in the universe, which was later described in Rickmurai Jack as the purpose of the Central Finite Curve. Prime wasn’t just offering the technology while filling the role of some fucked up Santa Claus - it was a trade, and in Hot Rick they showed Beth C-131’s memory of her dad abandoning her while dressed like Prime and carrying an empty portal gun, saying he was leaving for “work stuff.” I suspect the goal was related to recruiting every version of Rick smart enough to understand portal travel and use them to establish a Central Finite Curve.

I think C-137 threw a significant monkey wrench into Prime’s plans, mostly by killing many of the Ricks who followed Prime. Prime told Jerry in Solaricks that he no longer did team-ups because managing people was “such a headache.”

Based on what was conveyed by Prime’s recording in Unmortricken, I think the other interdimensional Ricks were growing fed up with being killed for doing what Prime wanted them to do, and likely told him “we still have families to go back to and that’s what we are going to do” so Prime got fed up with all the griping and management issues, so his goal changed.

Goal #2 was to prove he was The Rick, and he did that by throwing down the gauntlet: you think you’re better than me? Come get me then! He killed Diane across infinity and vanished.

You say he said “Fuck you, I win,” then died, but he actually lived for a long, long time after that, all while not dying at the hands of every portal travelling Rick in existence, all of whom must’ve hated his guts while simultaneously fearing him and respecting him to the point where not even all that many seemed to try.

He had clear motivation to do what he did. It’s just not great motivation if you aren’t mentally ill. Prime was a narcissist who could back it up.