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."
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"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).