because Sumi knows that Utou puts others above himself, Sumi felt that he did not deserve Utou and tried to reach the image that Utou has of him and stop depending on Utou, sadly that moment coincided with Shiratori manipulating him to hurt Utou. You can blame Sumi for hurting Utou (Sumi even feels extremilly gulty about that) but not being manipulated when someone was using his weaknesses against him in full awareness
i think sumi was just envious because of utou's relationship with the first omega...just see how his act was similar...he didn't think about utou...so he deserves it
that is also true, he was jealous and hurt, that worsened the things; but nobody deserves to be manipulated
And Sumi did think in Utou, he has shown several times to feel guilty, recognize Utou's kindness and not want to cause him problems
What omega has done was his fault. He played himself. Utou deserves better than that. Utou was the only one who fights for his love. He always put his loved ones before him and they took advantage of him. They broke his heart. At this point all I want is Utou to be happy. With someone else or not. He is the only one who deserves happiness