Katsuki

Izuku and Katsuki met as children - introduced to be each other's playmates - and they became close friends after sharing a lot of time together reading books and studying together. But one evening, when Katsuki walked through the halls of his home, he saw two men kissing and was at first, appalled, confused and then curious. He tried kissing Izuku since he did have some sort of affection for him. Gently kissing him on the cheek. However, his mother saw what he did and in a fit of anger and disgust, sent Izuku away; and years passed with Katsuki mulling over the loss of his best friend/love of this life. As a result Bakugo became obsessed and after years of waiting, he met Izuku again. Only find out that he was engaged to Uraraka. Izuku wasn't in love with Uraraka and just went along with her to erase any doubts or suspicion about him; he's merely being tolerant of Uraraka's "interest" in him. Courting her for formality. So Bakugo poisoned Uraraka's drink his jealousy taking over. Pretending to be the good guy for Izuku to lean on during her funeral Katsuki was happy to see her gone. He thought that Izuku would never feel the same way as he does and was afraid Izuku would fall in love with another woman at the ball. So in an act of desperation, he puts poison in Izuku’s drink. He poisons Izuku because "If I can't have you, no one will. So here's the poison you introduced to me when we were kids Izuku already knew beforehand because he's familiar with the poison and recognized it by smell but drank it nonetheless because he knew he "could take it". Katsuki caught Izuku and ran out of the ball with his limp body jumping off a balcony with Izuku in his arms. However later after he saw Izuku lying there just being motionless and unconscious, he immediately had the huge feeling of remorse and guilt. Thinking that he had ruined everything and the love of his life was gone. Then Izuku woke up and grabbed Katsuki pulling him into a hug.