Keep in mind, he's been away from her for how long?

You expect her to still even have feelings for him after that long of not even seeing him? I mean Kazumi was from Stage 3, which means over a year has passed. Itsuki needed to move on a long time ago. Which is the main reason he got so hurt.

Granted, it was fucked up she was about to use him like that, even knowing how he felt, but in a sense, he was too Naive.

Fault lies in both parties, or neither, not really Kazumi specifically