I think Aang would have the worst time of it in some ways, since Air Nomad culture was generally so non-violent and all; all his most basic moves are defensive, he doesn't even eat meat -- even if being the Avatar gives him a better view of the big picture, I can't see him ever being terribly happy about having to kill or even just hurt people.

Sokka might have the easiest time coping in some ways, since being a warrior is so central to his sense of identity and all -- but at the same time I think that might leave him suppressing any post-traumatic reactions since that wouldn't look manly? I'm thinking here of some military and police veterans I've known -- folks who didn't really talk about their wartime experiences for decades and decades after it happened, or folks who've literally been in "if you don't kill this person in the next second you and all your buddies are dead" situations, who did what they had to do, what they'd been trained to do, and still wound up really seriously broken up inside about it all. To get to the point where you can kill as callously at Jet you really have to strip away all empathy and see the other person as somehow less than human, and I think it would take a lot to reduce someone as grounded as Sokka to that point. Not saying that it couldn't happen, but I think it would have to be a long war-grinding-on-endlessly process like you mention.