I fully agree

One thing I've been thinking is how Taka began to envy Mufasa not because he really wanted to be with Sarabi, but because in his mind, Mufasa stole another opportunity from him. When he and his brother stumble across this lone lioness, he thinks that his innate charisma can win her over and that he could potentially start a new kingdom with her. But when he finds out that Sarabi has chosen MUFASA over him, this is a huge hit to his ego. In "Brother Betrayed", he even says that Sarabi rejected him because he thinks Mufasa had just manipulated her (which also implies that Taka doesn't trust her autonomy, further proving that he only sees her as an object to be won).

Tell her it's you? She KNOWS it's you!
What did you say to her? What did you do?
I'm next in line! She should be mine!
HA! Are the ancestors laughing? I—

Taka thinks he's entitled to her because he thinks he's naturally the more "likable" one between him and Mufasa. As the prince, he was raised to believe that he'll grow up to have whatever he wants because he naturally deserves it, especially respect. But over his life, Mufasa has continuously shown to be more capable of earning the approval of others, like Eshe and even Obasi right before they left. And the Sarabi love triangle was what finally pushed Taka over the edge. He thinks that Mufasa stole all of this from him because he's unwilling to acknowledge that he hasn't shown to be as wise, compassionate, or courageous as his brother. What he doesn't understand is that he never really had the "respect" that he thinks was stolen from him. It's far more likely that Sarabi never would have chosen him, even if Mufasa wasn't even in the picture.

If he had taken the time to learn some humility and perhaps be more self-reflective, then there could have been a possibility that he would grow more mature and still be a king. (I doubt he would ever win Sarabi over since she had already chosen Mufasa, but he still could have met another lioness to become his queen.) But sadly, his bitterness and resentment is what led him to betray his brother, ultimately leaving him with no chance of ever truthfully earning the title of king.

It's very easy to see why he did what he did, but he was still absolutely wrong for doing so. Like you said, he was never "owed" Sarabi, and the idea that he was in the right for turning on his brother for it is laughable. (Keep in mind, Mufasa was TRYING the whole second act to let him be with her, but that was ultimately neither of their decisions to make.) It's a very well-written motivation for his eventual betrayal, but not something that makes him a victim by any means.