"You dare suggest I betray Iroh? My first born? Directly after the demise of his only beloved son? I think Iroh has suffered enough. But you, your punishment has scarcely begun!" - Azulon
Ok. I know that some will say the "crazy" began with Sozin. To a certain extent, I would have to agree. Genocide does not come from a stable mind. But Sozin didn't start out that way. Megalomania (if it can be called that in this case) began after he became fire lord. Certainly the potential was there before but we saw no evidence of that. The FN was in a time of unprecedented prosperity. Skilled leadership brings that about.
If we move on to Azulon, other than continuing the war, I see no evidence in what he says or does. Of course there is that thing about which line to cross his own bloodline with. But it seems that he was genial enough to Ursa and her family. In ATLA flashbacks, Ursa appeared to respect Azulon. Prolly not so much once Zuko's life was about to be taken, though. And wow. Kill your own grandson? Cold. Really cold.
I don't think it's necessary to move on to Ozai. Ozai was even drawn with "crazy eye". Scary. Only the trips to Ember Island give any hint of normalcy and tender feelings. And Azula dutifully following in the footsteps of the patriarchs. But isn't it odd that the "Crazy" skipped Iroh (and Zuko if The Search concludes with him being a legitimate son of Ozai.)
I guess my curiousity is trying to work out when the barrel went over the falls, so to speak. And trying to weigh nature vs. nurture as it pertains to the fire lords.