Hello RiRi! I've been meaning to ask since you're like... A Sozin connoisseur to me the most (besides Marcy LOL) but would you say that Sozin must've been at least "slightly" jealous of Roku at some point or another? I suspected that Sozin was pretty much in deep denial that he can't be jealous of him, after all, he's quite excellent at everything he ever worked hard on. But overtime, the more he hears and sees that Roku is catching up and even doing far better than him ever since he held the title as an Avatar... He's a little sick of it even if he's also simultaneously proud of him as well,
Well there're two ways to answer this question. One is acknowledging the Ribay books and the other is ignoring them. Given I don't like the novels that much, I'll just give a quick summary of the answer if we keep in mind those events:
Yeah probably. Sozin thinks he's the bees knees and clearly believes he's superior to Roku in every way. Roku is like an endearing helpless individual he enjoys using to validate his own self worth, by making him dependent on him. Riveting.
Now as for show canon and my own two cents on that:
They made Roku and Sozin share a birthday for a reason. Some fans have even noticed Sozin demonstrated the four bending disciplines quite well in the bedroom scene; suggesting he had spent time studying them. Why? Is it because he thinks he has a shot at being the Avatar? Is it because he wanted to be the Avatar? If this is intentional subtext, then yes, Sozin being jealous is inevitable. He's just very good at hiding it because he's a clever person.
Would he like being jealous? No one likes being jealous/envious; it's misery. Maybe he even felt guilty about it. Would he try to ignore it? Very likely. Sozin may love Roku and feel happy for/feel pride towards him, but I don't see his more negative feelings being cancelled out because of that. The more you care about someone, the more likely you'll experience the whole spectrum of human emotion due to them. So yes, it's probable he was jealous.
I love that the episode doesn't wave it in your face, however (the most overt it gets, is his immediate reaction to finding out the news at the party). His betrayal feels more unexpected and resonant because Sozin took the "rejection" so well.
If you care about my opinion on the jealousy angle however, I don't think it's the most interesting thing. Sure, it can be a component of their relationship post-reveal, but the focus should be on Sozin's beliefs and how they jar with Roku's. Them arguing over philosophy/politics is more compelling, so that's why I don't tend to focus on the jealousy aspect in my fics. Sozin getting over it and finding his own path to importance is what I like about his character.
Hope you liked my answer.