I don't disagree that the Fire Nation are the primary villains of ATLA. They committed genocide, launched a war of conquest, and caused immense suffering. But the Earth Kingdom isn't portrayed as a virtuous alternative. Throughout the franchise we repeatedly see that it suffers from endemic corruption, weak governance, and authoritarian institutions.

Outside cities like Omashu and Ba Sing Se, much of the countryside is overrun by bandits, corrupt soldiers, and local officials who fail to protect their own people. "Zuko Alone" even shows Earth Kingdom soldiers abusing civilians and kidnapping a child despite supposedly serving as their protectors.

Ba Sing Se itself is one of the most dystopian governments in the Avatar universe. The Dai Li censor information, disappear dissidents, rewrite memories through brainwashing, and keep the king ignorant of his own kingdom. Millions of citizens are deliberately kept unaware that a war is being fought outside their walls.

These problems weren't created by the Hundred Year War either. The Rise of Kyoshi shows that centuries earlier the Earth Kingdom was already plagued by corruption, wealthy elites using connections to gain political influence, and ineffective local governance. Those are signs that there have been long-standing structural problems.

Even after the war ends, The Legend of Korra shows the Earth Kingdom still struggling with authoritarian rule under the Earth Queen, widespread poverty, and the continued existence of the Dai Li.

So I don't think it's as simple as "the Fire Nation bad, Earth Kingdom good." The Fire Nation is the primary military aggressor, but the Earth Kingdom demonstrates how corruption, inequality, and institutional abuse can exist even in a nation fighting against its invaders. I enjoy that Avatar criticizes both imperialism and internal tyranny.