I’d describe Ozai as Lawful Neutral. To me, a character has to have more than just a disregard for human life to be Evil- he has to ENJOY causing suffering and death. Ozai just wanted power for himself, but didn’t care either way if people got hurt or not in the process. Though perhaps towards the end, he might have decided on genocide out of a desire for destruction (brought about the Comet’s power surge?) So maybe he had evil tendencies. He might also have been faking having a respect for the Fire Nation’s laws and honor, in which case he’d be True Neutral. It’s hard to tell. (If he killed his own father, as I suspect, then he was definitely faking being honorable.)
Azula, on the other hand, is Neutral Evil. Even from childhood she enjoyed making others suffer, even if only emotionally. Again, telling if she was Lawful or Neutral is difficult because she could have been faking respect for the rules all along. Of course, once she snapped, she was definitely Chaotic.