Was Dolores Umbridge steered by the Imperius Charm in book 7?

Question

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.

Answer 1

Umbridge favours power, not ideology. She didn't need the Imperius Curse to behave wickedly.

Janoma's answer to the question of whether Umbridge was a Death Eater or not makes this point. It's important to understand Umbridge's motivations. She doesn't care much about which laws she's implementing or which regime she's propping up. She cares about being in a position of authority where she can intimidate the powerless and punish those breaking whatever the law happens to be at the time.

In Order of the Phoenix the Ministry is anti-Dumbledore so she devotes her efforts to countering Dumbledore and Dumbledore's Army. Her objective is to reclaim Hogwarts for the Ministry and to frustrate the work of those who are trying to proclaim the return of Voldemort. She takes a lot of pleasure in punishing Harry with her blood-quill and in forcing Dumbledore out of his job. But it's not out of commitment to the cause. It's because it makes her feel powerful.

So she sees no contradiction in Deathly Hallows in enforcing a completely separate set of rules. Here she's punishing Muggle-borns for their lack of wizarding heritage, as well as punishing petty criminals like Mundungus Fletcher. She doesn't care that the Ministry of Magic has undergone a 180 degree change of direction under Voldemort. She merely sees an opportunity to cement her position within the Ministry. Although she did have fairly unflattering views as to the place of Muggles (which probably extended to Muggle-born witches and wizards):

After a glass of sweet sherry, Dolores was always prone to spout very uncharitable views, and even those who were anti-Muggle found themselves shocked by some of Dolores's suggestions, behind closed doors, of the treatment that the non-magical community deserved.
_{(Pottermore, "Dolores Umbridge").}

That, combined with her lust for power, was why the Death Eaters were happy for her to keep her position under Pius Thicknesse.

When the Ministry was taken over by the puppet Minister Pius Thicknesse, and infiltrated by the Dark Lord's followers, Dolores was in her true element at last. Correctly judged, by senior Death Eaters, to have much more in common with them than she ever had with Albus Dumbledore, she not only retained her post but was given extra authority, becoming Head of the Muggle-born Registration Commission...
_{(Pottermore, "Dolores Umbridge").}

She has no morals at all so the use of the Imperius Curse is unnecessary. She will do whatever she can to maintain her power, regardless of what she is asked to do.

That's what makes her such a truly frightening character.

Answer 2

You can find the answer in Harry Potter and the Order of the Phoenix, said by Sirius when talking to Harry out of the fire.

"I doubt it," said Sirius. "I know her by reputation and I'm sure she’s no Death Eater —"
"She's foul enough to be one," said Harry darkly and Ron and Hermione nodded vigorously in agreement.

"Yes, but the world isn't split into good people and Death Eaters," said Sirius with a wry smile. "I know she's a nasty piece of work, though — you should hear Remus talk about her."

[...]

"Apparently she loathes part-humans; she campaigned to have merpeople rounded up and tagged last year too. Imagine wasting your time and energy persecuting merpeople...."

She hates part humans, or mudbloods herself. And she just got opportunity to do as she likes under the new regime (in Deathly Hallows).

Answer 3

Umbridge did not favour You-know-who’s views, she was just a snake who wanted power and would side with anyone who had the most of it. You-know-who seemed to be most powerful at the time and he controlled the ministry , so she sided with him. Also, it was an opportunity to accuse any enemies she had of being mudbloods.