Well really, it kinda makes sense. One of the biggest signs of a person's psychopathy would be an inability to feel their own or other people's emotions. Flowey's inability to take in the love of the people around him, or to give love (or anything, really) is probably as good a sign as any that his natural state is psychopathy.

Gee who would've figured the first thing Asriel does when he returns to being Fowey is to be a huge shit to everyone who he now totally remembers.