"War criminal" is, as far as I'm aware, a relatively recent concept, and almost certainly something that the setting of the first series of Avatar wouldn't have had. There weren't any "rules of engagement", you were basically free to do as you liked. Especially Asian civilizations... they may have had ideas of "good conduct" and your lord might get on you if you made him look bad but there was no concept of a 'war criminal' because for one thing there was generally nothing like an international body or a treaty that would govern such a thing. Who exactly would try Iroh for his "war crimes"? The Earth Kingdom? Seems a little biased.
Back in the day if you didn't want your people murdered and raped, you tried to strike a really good surrender deal with the opposing force and then hoped they stuck to it and could actually restrain their men. And even if they didn't feel like it, oh well, that guy's got a reputation for reneging but if you want justice you better look in the dictionary between "jackass" and "Kavorkian" because ain't no international court to scold the Huns.