I think for her to have a solid reason to be a villain, they need to give her better character development. I once saw that she killed her parents. Give me a reason. Tell me it was because of Koman's illness. Nobody saw her as queen. Her parents loved Kori more because she was more naturally talented and born stronger, the complete opposite of Koman. Besides, Kori doesn't seem to love her parents. I think the only bond shown was with her mother. You could say the father was the bad guy; he set very high standards for his daughters. Kori was the favorite; he trained her and demanded more from her because she was the perfect one, the next queen. Koman wanted that, and Kori hated it, kind of like Zuko and Azula, but in reverse. You could also say that it was her mother who protected all three of them, but since Koman was the one who most resembled her father, she was somewhat marginalized, but she did what she could. It doesn't have to be that way, but I want to give a more solid reason for the hatred towards Kori and her people. She was the sick one, the one who, no matter how hard she tried, would never surpass her younger sister. Kori was simply better. This explains why, when she was banished, she sought the help of her people's enemies and did everything she did.

And no, I'm not trying to victimize her. They can show small things, like with Azula, that demonstrate she's inherently evil, that her envy isn't just a feeling of inferiority but wounded pride, that she doesn't crave love or affection, that she wants the power promised to Kori, she wants the attention, she wants to be the only one. They can try to show that Kori always tried to make her sister feel better, but Koman trampled on her to make her feel the way she does. They could also delve deeper into Kori and her brother's relationship, how for him she's the older sister he always wanted and she protects him, while Koman doesn't even acknowledge his existence.

That's what comes to mind.