She's lashing out at someone who looks like an easy target, while her life is falling apart. It's "revenge" against life in general, but the target is some poor sap who couldn't get into her school. From the previous chapters, none of the characters knew the relationships between each other possibly until Aya made the connection during the amusement park date. At that point, I think Yuu and Aya were broken up. Also, we don't know that Yuu's feelings for Hikari are necessarily romantic, and at what points he felt which way. Given the thematic naming, I think we're gonna get a Yuu arc after Aya's turn, to give the full picture of what's going on.
It lines up with what he told Hikari after the summer festival, and she didn't take it as a romantic confession. It could be that their feelings never matched up. Yuu probably changed after going out with Aya, and Hikari fell for that version of Yuu.
Your mistake was assuming she would be, like, a character with traits and flaws and shit. Unfortunately we're getting a cardboard cutout.
