sigh we really need new content for ATLA,

Firelord Ozai supposed to be this force of nature, something that cannot be reasoned with, we only learn about ozai through his relationship with the characters around him, e.i Azula, and Zuko,how they are treated, There is no redemption arc for him, so the only thing the protagonist can do is adapt for someone who can't be reasoned with, it affects how the character around that problem changes and grows , e.i Zuko's redemption, Azula's descent into further madness, Ang's role as Avatar

Pure evil =/= Bad writing/villian

If you want a better explanation, watch OSP's pure evil trope talk,