Trump is a psychopath. When he finds out someone loves something, is dedicated or loyal to something, has ideals, is fighting for something they value, he considers it a weakness to be exploited. He realizes he can hurt people by trashing what they love and cherish. Zelensky loves his country and his people, he fights for it with his life, so for Trump, that is an invitation to trash Ukraine and he enjoys it, he feels it gives him power.