Everyone talks about Zuko's redemption arc but nobody talks about Iroh's redemption arc. Probably because it's a lot subtler and we only see it from the outside, and only the tail-end of it.
Before his son died, Iroh was a pretty bad dude. He had redeeming qualities, like sparing the last dragons and loving his family. But he was still a general leading a campaign of conquest.
His redemption started after his son died. He clearly had a lot of self reflection as well as a spiritual awakening in the following years.
But he was still conflicted, trying to remain loyal to his nation and family while also practicing the principles he learned from his travels. But those things were incompatible.
Even after he had to actively fight against the Fire Nation when Zhao threatened the moon spirit, he still thought he could return to living in the Fire Nation.
It's only after Azula tried to imprison him and Zuko and forced them on the run that he gave up on the idea.
I think it's sometime around their time staying in Ba Sing Se that he finally decided to oppose the Fire Nation. At the price of his own freedom, eventually.
I think seeing Zuko's struggles with redemption reminded him of his own earlier struggles, and it's what finally pushed him over the edge. He didn't just help Zuko redeem himself, Zuko helped him redeem himself too.
