NOTE: I don't have any idea which side Umbridge favoured: the 'dark' side or the 'good' side.

Between book 5 and book 7, the Ministry of Magic was infiltrated by Voldemort. For me, it just seems perplexing of how the situation occurred.

In the book 5 Umbridge's attitude, alongside that of most of the Ministry employees and Minister of Magic (Fudge) was said to be illuminating. They strive to tell Hogwarts students that the claim that Voldemort is back is a total lie.

In the book 7, Umbridge was said to hate the Mudbloods. Of course it's harrowing that the Ministry was favoring the bad guys (I mean most of them and because they are infiltrated). But it's confusing how Umbridge deals with the situation. Umbridge favors each and every wicked thing that's happening in the wizarding world, particularly at Hogwarts.

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

Correct me if I'm mistaken about something.