Yeah there was the first person to teach Aang fire bending, that super reluctant guy who was all "fire bending by it's nature is destructive and will burn all who use it". There was Uncle Iroh, there was Zuko, who even when he was deeply in the Fire Nation koolaid, was a very good person. His empathy and compassion for his people are the thing that got him burned and exiled. There was Zuko's mother, who was presented as being a normal, nice person (as best as I recall). There was Sokka's instructor like you mentioned, and in theory, every other fire bender in the White Lotus Society. So it's hardly without precedent.
The issue I think, is that it's like trying to find a "good nazi" when the majority of those you see, are card carrying, dyed in the wool, nazis. Sure some of them were probably just going along with it, or just not engaging in activities they didn't agree with, but THEY KNEW, better than anyone, what happened to those who defied the will of the nazi party. Same goes for the Fire Nation. I mean The Firelord burned his own crown prince, in public! A Shameful Display! And kicked his ass to the curb for defying him! You think Random Fire Nation Mook 27 is going to make much of a racket if he didn't agree with the Fire Nation policy?
And it's important to remember, that the Fire Nation, isn't the same thing as just fire bending itself. There was that entire arc about Zuko learning the "true" fire bending, and how it wasn't how the Fire Nation, had bastardized the philosophy behind their bending.
Also it's important to remember, that the Fire Nation was, by nature of the narrative, framed as the Bad Guys. And they were, but you can't spend a lot of screen time, un-villainizing your villains, at least not on a large scale. You have a few representative characters, that you slowly, over the course of several seasons, show to be actual people, and not mindless killing machines. Or you have one who is like that from the start (Uncle Iroh), but clearly have him be marked as an outsider to the mainstream way of the villains. Then, when you get to the final season, that more directly involves that faction, sure, you can flesh it out in more detail. Which they did. It's just the random fire nation people aren't the MCs, so by necessity, they're going to be small glimpses, that represent a larger, unseen division.