There was much satisfaction to be had in baiting Katara. Especially when – for once – Aang was taking Sokka's side.
The two of them imitated "spirits" for a few minutes, until Katara stalked off. Sokka watched in satisfaction.
"Sokka," said Aang tentatively, "don't you think . . . she's got a point? I mean, these people do need help. And, well . . ."
Sokka understood. But . . . he didn't see what they could do. And really, by saving the world they would help these people, too.
After he explained this, Aang seemed relieved. He nodded slowly. "Thanks, Sokka."
This is set when Aang and Sokka start being obnoxious about spirit magic. And I really can't imagine Aang honestly thinking, "Oh, whatever, these people are going to die. Meh." But I do see him deciding for the greater good, with Sokka's help.