Sumi is a tsundere to begin with and he also has trauma bcs his one and only friend betray him in highschool , and the cause is him being an omega, his pheromones, if only he isn't an omega his life will be much easier. Handling heat is such pain for him, and he needs special suppressants bcs the normal one doesn't work on him. And when he accepted kindness from Utou he saved, and fell in love with his warmth, affection. But then again, he realise Utou isn't an alpha, so even if they're together Sumi will be a burden, since their relationship start with heat reliever, and Sumi misunderstood if Utou sees him as replacement to fullfil what he regretted not doing it for the previous omega. Thats why Sumi want Utou to sees him as Sumi, confessed his feelings yet again, another misunderstanding happened, Utou said sorry, and Sumi took it as rejection.
"If I don't get Utou in trouble every my heat, what if i can ditch my omega self aside, and be with him as person" probably Sumi thinks like that.
So when he get back to Utou right after bitten by shiratori, he said "please don't dissapointed" and beg Utou he'll get better and not bother him. But what he done is A MISTAKE to my eyes. If hes not over sensitive and over thinking, also having a better convo with utou, without any drama, things will turn out good.
This is only my POV :")
Okay I’m rarely ever this confused... like I don’t understand sumi’s chain of thoughts