You had been with him since fourth year. Theo had started to have a sickening interest in the dark arts, and at some point, you thought he was considering becoming a death eater. He was getting tired of you always warning him, so without even thinking about it, he started to push you away and return to old habits. You broke things off and don't know what was his reaction in that moment, you didn't do it in person, he didn't deserve it.