Issue is about not having any security guarantees. Problem with dictatorships is that they won't ever stop. The deal is beneficial for Russia. Russia will see it as a win which will encourage them to do the same thing every few years on repeat. Territory is one of the few limited resources in the World. Dictators desperately crave for it. Any economical damage they think is temporary and they see it as an investment. They certainly don't feel economy being bad. But they will feel powerful, but gaining territory. Only deal Zelensky should sign is one which will ensure that Russia doesn't invade Ukraine or any other countries again. Any other deal is a fake peace. Everyone around Russia and who has experience with Russia knows this. E.g. ex soviet countries.
I think the whole thing was a setup, and while Zelensky made mistakes, even with perfect behaviour, it was bound to go in this direction since the deal presented was not something Zelensky or a person with democratic values should accept, and Trump's goal was to humiliate him into submission from the get go.
Consider how much provocation Trump and Vance had done before that, calling Zelensky a dictator, the patience the man had is already amazing to last so long. But they would've kept provoking him until he says something which they can escalate on to humiliation.