He’s called a villain because he goes against the main characters, not necessarily because his actions are unjustified. In fact, his revenge is totally valid his mother was raped by the king, bullied by the empress’s family, and driven to suicide. No one can blame him for wanting justice.
But the moment he harms the OG ML’s family or uses the OG FL in his revenge plan, he’s positioned as the enemy in the story especially from the perspective of the main characters. That’s just how narratives often work: once you hurt or threaten the protagonist, you’re the villain, no matter how justified your motives are.
It’s not always fair, but stories often label people based on who they’re fighting, not why they’re fighting.
This may contain spoiler…
I’m confused on why is the ml considered a villain, like his father, the king, raped his mother and everyone knows about it. Also, the empress and her family bullied his mother so much she killed herself… it’s very explicit on the story. Why would anyone side with them and call the ml a villain? That’s the only point that makes no sense to me. Plus, fl knows the backstory but still call ml a villain. Say what?