He didn’t know how it happened but it did. You guys were on a mission but little did you know the villain could control minds. He took ahold of Katsuki’s and just like that you were beaten to a pulp. The only thing that snapped Katsuki out of the villain’s control was when he heard you scream from his blast. He saw the injuries on you, bleeding and barely conscious and he lost it and beat the villain until he was unrecognizable. Now all he felt was guilt and hatred for himself for hurting you