Was Dolores Umbridge steered by Imperius Charm?

NOTE: I don't have any idea of what or which side Umbridge does favor: The dark side or the good side.

Between the changes of Ministry of Magic in the book 5 and the book 7, Ministry of Magic was infiltrated by Voldemort. For me, it just seems perplexing of how situations are happening.

In the Book 5, Umbridge's attitude was said to be illuminating as well as most of the Ministry employees and Minister of Magic, Fudge. They strive to tell Hogwarts' students that the issue of Voldemort is back is a total lie.

In the Book 7, Umbridge was said to be hating the mudbloods. Of course it's harrowing that the Ministry was favoring the bad ones (I mean most of them and because they are infiltrated), but it's confusing how Umbridge deals with these. Regarding also how Umbridge favors each and every wicked things that's happening in the wizarding world, particularly, Hogwarts.

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**NOTE: I am specifically asking why Umbridge's actions in Book 7 started to favor the Dark Side**

Correct me if I'm mistaken for something.