Think of all the technological progress that was made during their conquest of the world. During the duration of the series, the Fire Nation was the only nation to use machinery and steam power. Then, by the time Korra comes around, all the tribes and nations suddenly have it. Maybe this technology wouldn't have been possible without the Fire Nation.
TL;DR
The Fire Nation was bad, but made significant technological progress.
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They were like the Americans. Evil bastards that came in steam ships for invasion.