Harry Potter and the Order of the Phoenix
How we cite our quotes:
The upshot of it all was that Professor Umbridge spent her first afternoon as Headmistress running all over the school answering the summonses of other teachers, none of whom seemed able to rid their rooms of the fireworks without her. When the final bell rang and they were heading back to Gryffindor Tower with their bags, Harry saw, with immense satisfaction, a disheveled and soot-blackened Umbridge tottering sweaty-faced from Professor Flitwick's classroom. (28.99)
Professor Umbridge has been working hard to seize ultimate authority at Hogwarts and now, after getting Dumbledore dismissed from the school, she finally finds the power she's been seeking. But we also see the unexpected drawback with being a power-hungry control-freak. If she keeps insisting that the teachers respect her authority, then they have the leeway to leave everything of importance – including rogue fireworks in classrooms – to Professor Umbridge's personal attention. In short, if you're not willing to delegate and allow other people to do their jobs, you may find yourself "tottering sweaty-faced" from running around doing other people's jobs.
"The Cruciatus Curse ought to loosen your tongue," said Umbridge quietly.
"No!" shrieked Hermione. "Professor Umbridge – it's illegal."
But Umbridge took no notice. There was a nasty, eager, excited look on her face that Harry had never seen before. (32.197-199)
Here's the moment when we really see that all of the rules Professor Umbridge pretends to care about don't mean a d--n compared to her pleasure in gaining power (physical, emotional, whatever) over other people. She's willing to break the Ministry's laws – and she's done it before, by ordering two dementors to Little Whinging to attack Harry Potter. Professor Umbridge isn't a fanatical believer, the way Death Eaters like Bellatrix Lestrange are. She's just attracted to authority because it gets her what she wants: power over weaker people.
"Fine," said Hermione, now sobbing into her hands again. "Fine ... let them see [the weapon], I hope they use it on you! In fact, I wish you'd invite loads and loads of people to come and see! Th - that would serve you right – oh, I'd love it if the wh - whole school knew where it was, and how to u - use it, and then if you annoy any of them they'll be able to s - sort you out!"
These words had a powerful impact on Umbridge: she glanced swiftly and suspiciously around at her Inquisitorial Squad, her bulging eyes resting for a moment on Malfoy, who was too slow to disguise the look of eagerness and greed that had appeared on his face. (32.231-232)
Professor Umbridge manages to draw people like Draco and Argus Filch to her side because she lets them do what they've always wanted to do: bully other people freely, without being punished. Draco loves being able to lord it over the other prefects, and especially the Gryffindors. And Filch is happy at last to be allowed to whip the students as he has always dreamed. But they're not connected by the bonds of loyalty and friendship. Professor Umbridge knows that Draco would double-cross her in a hot second if he thought he could profit by it. So, she has no one to rely on at all – a fear that Hermione plays on in this scene, when she invents a super-weapon to lure Professor Umbridge alone into the Forbidden Forest.