Have a lot of mixed feelings on this. Umi definitely mistreated Ame in their teen years, never reciprocating her love and care... but he couldn't help how he felt. I mean, I could barely imagine the despair if my dad offed himself after my mom abandoned us... it's a difficult situation
I also still have a hard time feeling that she is entitled to his romantic affection even in spite all she's done for him. If he can't be happy with her, because he doesn't swing that way or whatever, I'm not sure she has the right to get in the way of that. However, Itsuki seems to have several screws lose and it genuinely could've been he somehow hurt or manipulated Umi. I mean, his disgust was so intense when he first remembered getting with Itsuki, and there was 'something he never got to say' to him, which could've been anything from "I want you in my life forever" to "stay away from me you stalking gaslighting freak". Maybe Ame is the good guy and her initial yandere shtick was misdirection.