I'm always surprised how many people fail to understand betrayal, especially for a kid.
Keisuke was alone, he had almost no social contact, and was extremely vulnerable. His identity relied on having at least someone at school who proved his value as a person. And when a fragile person is hit directly at their weak point, or a child is hurt by someone in a place of trust, losing that foothold or pillar of support causes a total upheaval of daily life, which in turn causes trauma because their brain is forced to operate at extremes. He was at peak love hormones with nothing else to show for himself, barely convinced he wasn't worthless, and took a heavy blow. He's a brat, but at least the trauma is slightly realistic.
It's lazy reading to take Hebikawa at her word either. Of course she would see herself as innocent, they were such a small part of each other's lives and she holds him to the same standard of people who had support systems. How can you not see past rote victim-blaming? Her transactional mindset and way of evaluating everyone's worth means she's likely to hurt or push away people who can't navigate that. Kusunoki's innocent friendship seems to have struck a chord with her, somewhere. I find Hebikawa's interactions with Keisuke annoying or deranged, but she's not a psychopath, just weak and prideful.