He doesn't say things like that, and he didn't mean to just now.

Seeing that he's hurt her, his scowl melts into a regretful frown and he backs down almost immediately.

"I know you aren't, Liz. I just ... " He turns his head away from her and studies the fingers of his stone hand along the side of the bed, going silent as he works out his words.

"I don't like him. And, I don't trust him."

There, he admitted it. Part of the problem anyways. The other half being the reasons he doesn't like Zuko aren't going to be voiced aloud. Mainly because it's about Red being insecure and not happy someone can help her where he can't.

"You really want to do this?" She doesn't need his permission, he knows that. And he's not really asking to give it but he wants to be sure like she is.

This answer won't do it, but maybe it'll bring him closer to that point.