system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why were Dementors never used against Voldemort and the Death Eaters?

If they are under the control of the Ministry, and the only way to withstand them is a Patronus that evil people can't conjure, they seem like they would be ideal for countering dark wizards.
gpt
Dementors feed on happiness. Voldemort promised that they could feed on all the happy wizards and Muggles they wanted if they joined him and the Death Eater. All the Ministry could do was send them after bitter, angry wizards who would not be enjoyable to feed on.