There's something to be said about an angry water-bender. Watching Katara yell at her brother Sokka, Zuko isn't quite sure what it is.
Maybe it's the fact that they look rather funny (but Zuko's not stupid- and he's not Toph- so he doesn't laugh when she yells and he never mentions his suspicion) when they're so out of their element- because it's fire-benders that get angry, right?
Perhaps it's the fact that they're humanized when they're livid. They lose that calm, sweet, serene look about them and start acting like real people. Hell, they are scary (not that Zuko could ever be scared by a girl or a water-bender) and Katara is perhaps the scariest.
Mayhap it's the fact that they are so sexy when their breathing is heavy, their nostrils are flared, their eyes wild, their replies like venom and they look ready to bite off someone's head.
There's something to be said about an angry water-bender, but Zuko isn't sure it applies to all of them.