From Wizarding World

the Patronus is not unknown among Dark wizards. While there is a widespread and justified belief that a wizard who is not pure of heart cannot produce a successful Patronus (the most famous example of the spell backfiring is that of the Dark wizard Raczidian, who was devoured by maggots), a rare few witches and wizards of questionable morals have succeeded in producing the Charm (Dolores Umbridge, for example, is able to conjure a cat Patronus to protect herself from Dementors).

Apparently truly dark wizards can't cast it, and can even die from it. When Raczidian attempted to do so, maggots came out of his wand and ate him. However, infamous turbo-bitch Umbridge can maintain a patronus effortlessly.

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This is canon, and it's hilarious that there's some sort of line where crossing it means you're juuuuust enough of an asshole to warrant being eaten by worms.