Of course she doesn't owe it to him to be nice. But don't be nice to someone and then talk shit about them behind their back. That makes you a shitty two-faced person.
I'm not saying she's a super villain; of course she isn't. But all these comments of "dude she's really not that bad", what? This chapter has just reinforced what we already knew, that she was two-faced in middle school. Whether the MC's trauma or extreme reactions to her are justified is a totally different issue. The fact is, she was two-faced towards him in middle school. And she's not apologising for that. She said it herself that she doesn't even remember what she said. So she was two-faced and considers that such unremarkable behaviour that it's not worth remembering. Definitely a much shittier person than Kusunoki, and worse than MC.