
Wdym stupid? She wakes up after centuries, on the verge of death, and finds out her lover not only locked her up and killed all her sisters but also bad-mouthed their legacy and gave them a slur. She has nothing left to do, only this intense resentment for a man who's long gone with a prestine and powerful legacy that was only built on her hard work. And look how grateful he's been.
Violence begets violence, a classic theme.
Objectively yes, very bad, she's not the moral person here and should know better. But why would she give a damn about morals with all these feelings and the knowledge that she's gonna crook any day now? If anything she's a great villain for bringing shades of gray to the story. The 1st Emperor isn't great nor just a goofball, he did bad shit too. Anita isn't simply the bad guy villain but a person that's become part of a circle of violence desperate to express her agony and anger.

My thoughts exactly, even Regina somewhat understands Anita's reasons for her revenge. She's actually a well written villain.
And, resentment, especially deep rooted ones cannot be easily get rid of just because the person or something that is their subject of grudge no longer exist before they get their closure. They tend to subconsciously look for someone who fits their ideal new target of revenge. It's sad but it actually happens a lot in real life. That's why psychologists and therapists exist because it's not easy to manage our emotions on our own when our emotions are just too intense for us to handle.

The thing is, she's the only magician left and all of her kin were wiped out. She no longer holds someone or something dear to hear. For her, she has nothing left to lose so having a positive relationship for the current empire is pretty much useless for her. And since the current thriving empire is the result of the lives of her Kin being annihilated of course that would only fuel her agony and grievances. For her, her only mission left is to erase the traces of the the man who betrayed her which is his descendants and the empire, not to mention that these descendants also have the blessings of the Gods that ordered the first emperor to kill her kin. So Regina, the current emperor, and Theor is not only the "traces" of the first emperor but also the Gods which is the enemy of the magicians.
So her whole villan arch is to take revenge on people who didn't do shit to her and her hatred is valid but not to the point of taking it out on the innocent. I mean, that just shows she's no better.
Plus she could've made a stronger relationship with the current empire and make it easier for her kind.
What kinda stupid plot for a villain.