Because he was backed into a corner where his pride was threatened and he instinctively lashed out like he always has. He was raised to believe that he isn’t truly worth anything without the “perfect” mate (which he was told was Dojun, so he has become fixated on him). So when he had to “defend” his actions, he used the same argument he used at the time. I don’t think he had ever really reflected or thought on the damage those actions caused before, because they only affected people he wanted to punish or control before. This is the first time that they affected someone who he felt loved by. So he had no idea that he was saying the wrong thing.
YEEES!! :(
I don't think Chowon still loves Dojun. He obviously likes Kyungsoo, I don't know why did he said that but I know he really don't mean it. In the first place we know how fucked up Chowon is, he's self-destructive. Maybe because when he grew up no one really pay attention to him that's why this is the outcome. And what he did to Heeso, after that and the humiliation he get I can't blame him altho he's at fault too.
Sorry this is so long, but I really feel bad for Chowon. Everyone deserves to be loved. :)