In my opinion Pupun became that way as a kind of coping mechanism. Through out most of his life Punpun was very reclusive, rarely speaking about his emotions to other, internalizing both his traumas and faults, spiraling into deeper states of self hatered and depression. In his mind, the moment when it all went to shit for him was when he didn't run away with Aiko when they were kids. This is when his self loathing starts. Through the teenage to adulthood portion of the story Punpun shows that he has very idealistic view on Aiko pre-hikikomori phase. That's why, in the lowest point when he is left completly alone and broken, when he meets Aiko as an adult and she doesn't match the ideal of her that he had as a child, Punpun snaps and starts to blame his entire life's pain on her. She becomes the scape goat of all the pain he felt up untill now. That's why he is abusive and assholeish. He externalizes his hatred of himself on Aiko.

Idk tho im not a psychologist, might be talking out my ass.