Zuko doesn't know what Toph's doing. Out of everyone there, of course Aang has a right to hurt him. He's a Riha - his people are the one's being targeted. He, out of all of them, has a right to express his emotions - to hurt him, to beat him, to break him, to kill him, to… And he's grateful that Aang's showing his true emotion, instead of pretending he's okay until later.
Because he knows now that he will never be okay. He'll never be accepted into this group. He doesn't even have a chance. He's not sure if he should have hidden the information - it would have kept him safer, but would have increased his humiliation ten-fold. And they needed to know. He tells himself this as Aang finally reaches him. They needed to know.
Zuko's not sure if he deserves to fight back, so he doesn't. He just sits there, while Toph yells at him. Eventually, she manages to get Aang off him, and he stares into the fire while she goes on and on, about what were you doing? How could you just attack him? Are you insane?
Zuko doesn't understand why she's asking. It's what they should all be doing - he knows it, and so does everyone else - they're silently watching the struggle. They aren't doing anything because they know he's - they know he's a crigel. They know he doesn't belong with them.
Suddenly, Aang stops fighting back, and Toph lets him go. He's crying, and Zuko's not sure why. He doesn't look like he'll get up for a while. Like a punch in the gut, he's reminded that he did this. Whatever happened to Aang is his fault. Katara gets up to help Aang, and she looks afraid. Of him, probably. He turns away. "I'm sorry. I'll go now."
And he leaves, but Toph follows him. "Hey," she says, "what's up?"
Zuko doesn't know how to respond to that, because he's not exactly sure what's up. "The sky. It's an horrendous shade of orange, and soon, it'll be dark enough that we can't see."
Toph's mouth twitches, but she doesn't quite smile. "I've never seen Aang act like that before. Not even in battle."
"I don't know what I did."
"Well, you know. Maybe he's just a Riha, and that's how they act?"
Zuko stops to stare at her. There's no way she just said that. She grins. "Good. You passed the test. Now I know you're not secretly beating him up."
Zuko can't smile, even though he knows he's supposed to. "Yeah. I just happen to know the one who's literally targeting his family."
She sighs. "Y'know, we're pretty far from camp. Wanna sit and talk? We can go back in a bit."
"Okay." So. They sit. And talk.