For over a year people have been speculating that something really bad is going to happen to Iroh and THAT will be the turning point for Zuko.
And tonight we see Zuko apparently betray Iroh . . . allowing Iroh to fall into the merciless hands of the fire nation and Ozai.
These two ideas work perfectly together. This is not Zuko going back on a season of character development but rather the set-up for the last straw . . . the final dramatic jolt that will break the metaphorical camel's back and turn Zuko against his own heritage.
Consider the Fire Nation. They are ruthless. They are ruled by a tyrant to in Iroh's words "is NOT the understanding type."
And consider Iroh. In the eyes of the Fire Nation he is a traitor. The once great Dragon of the West and the very brother of the Fire Lord defending the enemy and attacking Fire Nation royalty. How do you think this ruthless nation lead by the not-understanding type is going to treat Zuko's beloved uncle who, as proven in The Spirit World, he loves more then even his honor and/or throne. (He proved this by turning his back when he saw the Avatar to go save his uncle)
Iroh is going to be violently dragged to a prison cell to start with. There, he will lose his freedom and suffer as a prisoner of war, left to rot in jail. And that's if the FN is merciful . . . which we all know they are not.
Iroh may be tortured . . . prisoners of war have a history of having atrocities against humanity against them; starved until they're forced to eat the gaurd's dog, becoming outlets for gaurd sadism, prison gaurds even being punished if they treat a prisoner with an ounce of dignity. Heck, Ozai might demand Zuko torture Iroh himself to prove his loyalty to the FN.
Or, even more likely, Iroh may be sentenced to death.
This could finally do it for Zuko. He will be done being torn between good or evil, as he will be suddenly and violently torn to the side against the FN. He will himself be tortured by the guilt of getting his uncle into such a situation. I think will give them that final dose of maturing life experience makes him realize; he must control his own destiny, his destiny must not control him