<p>Eeeehhh...
</p><p>Ok, so it does seem a little out of character. But for the sake of argument, Katara, Sokka, and Zuko couldn't have been involved in every part of the prison-building process. Or -- how about this -- when the red lotus were captured, each prisoner was probably delageted to members of the Gaang. Katara could have handled Ghazan, explaining why his prison at least appeared to be the most pleasant. Zuko, on the other hand, could have been assigned Ming-Hua. Zuko, as we know, can sometimes be very harsh and unforgiving.
</p><p>For example: Imagine a Zuko hardened by the stresses of ruling and rebranding an entire country -- still haunted by the death of his best friend -- who just took down some very powerful criminals trying to kidnap baby Korra, in whom Zuko probably can only see Aang. I could very easily see Zuko saying "I'm gonna throw you into a volcano." Katara, of course, would stay his hand saying "What Would Aang Do?" so Zuko did the next "best" thing.
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