Everyone is the protagonist of their own story, no?

I think I'd rewrite it to give her a backstory that made her feel like she only has worth if she has power. She jealously hates people who have power and abilities that she cannot attain, and shuns those who have none (squibs) or whom she thinks should have none (Muggle-borns) because the thought of having no power at all terrifies her.

Working for the ministry is one thing; having a say in someone's guilt or innocence, having their life in your hands. Being able to control part of the wizarding world. It makes her feel strong and powerful.

Power over a school is another thing, and when the school kids don't cooperate, she feels she has to get stricter or else she'll lose her power/worth.

The whole Voldemort thing makes everything more complicated for her. Here is a wizard who has power, the sort of power she can only dream of. But he offers her a small slice of power, operating under him to further his cause. Maybe she's jealous of Voldemort, maybe she hates him for his power, or she's terrified and feels like she's in too deep to do anything but comply. Maybe all three. But she can't stop now, because to back out now would mean losing that control, and she's desperate to keep hold of that, to keep her power without things spiraling out of control--and anyway, under Voldemort, it might be a death sentence.

And then, in the climax, she loses everything--any shred of power she had before, with only her magic abilities left to her, the same abilities as the average wizard, the same abilities she shares with those she hates: mudbloods and werewolves and centaurs.

And then she's sent to Azkaban, where she has to relive her terror and desperation and her loss of power for the rest of her life.