Simply put, Trump wants Zelensky to give territorry to Russia, whereas Zelensky cannot.
Therefore Trump is using all means at his disposal to force him to.
Zelensky is right, the criminal shouldn't be rewarded. If he does, the chances are he will recover and strike again.
don't mind my username, that was more than 10 years ago, I'm a different person now, amazing how people change ^_^