I make an excuse cause we don't really know anything about her past and what motivated her to be a villain
No, we do know why she is the way she is. She was effectively the same as Kusunoki.
When she transferred into a new primary school, she tried her best to be nice and kind and get along with everyone. She tried to emulate an idol that she really admired, who was a bright and sunny person that brought smiles to everyone around her.
But, because she tried to be mindful of the feelings of everyone and to please everyone, she invariably made enemies of some people by befriending others. When she was nice to boys, they would swoon over her, and that would tick off the girls in her class. When she said she was worried one girl would get in trouble for wearing makeup to school (which was against the rules), the girl got mad, and they started shunning her. As Hebikawa dealt with this, she started getting annoyed, and thinking mean things about her classmates--something that she saw as "drifting from the ideal she embodied", which was distressing.
Then, her friend with the makeup accused her of tattling to the teacher--which she didn't do, and all her friends left her and broke off their friendship. And at that point, Hebikawa snapped, saying those hurtful things out loud to her ex-friend's face, and putting her down and making her cry. Hebikawa relished in the pain she caused, deciding that if others were going to be mean to her for no good reason, then she would simply be mean and hurtful first, to protect herself.
Shortly after, Hebikawa saw that the idol she revered for being so nice and kind got in major trouble over underage drinking and other illicit acts and was kicked out of the idol program--and Hebikawa came to the conclusion that anyone who was showing kindness or had a bubbly & bright personality was simply faking it--that no one would be that way for real because all it would do is get you hurt by others, and that it was all an illusion meant to manipulate.
And thus she became the way she is, right up until she met Kusunoki, who
actually embodies the things that Hebikawa used to hold as good traits to have. And that's why Hebikawa is so obsessed with Kusunoki and "unmasking" her--because if Kusunoki
doesn't have a Dark Side to her, then it means Hebikawa's entire ideology that she's had since she was a gradeschooler is false, which would shatter her sense of identity and mean she's really just a mean person who
could have been better, if she'd tried a little harder.
And all of that is outlined in Chapter 31.