[He doesn't mean it the way Kamui thinks he does, not thinking even for a second that his twin might take it to mean something like that. This argument, this situation is what he can't handle right now, not when he's too upset, too tired, too broken to even think of it properly. He loves you, and he loves Seishirou... why can't he be allowed to love both of you? Why can't the two people he cares about the most coexist peacefully?]
Kamui... [He looks down, his tone dropping all of its anger, only sadness and exhaustion remaining.] Please. Just... let me be.
[He doesn't want to fight you over this, he doesn't want to argue with you-- and he knows, if they continue this, he'll say more things he'll eventually regret. He loves you, so much it hurts to think of it, and that's why he doesn't want to hurt you any more by being upset and careless with his words. He can't do this, can't bear the thought of causing you any more pain than he already has. This... it's still him protecting you.
He doesn't say any of this aloud, though, because he doesn't realize how his brother could take these words, doesn't understand there's already been a misinterpretion with his earlier words.]