Umbridge is one of those characters the reader is supposed to hate and despise, but she isn’t a Death Eater, she’s just a nasty person. Sirius sums this up well by saying “The world isn’t split into good people and Death Eaters”, and Umbridge is the perfect embodiment of this maxim, showing the nuance that Rowling can give to her characters. Not everything is black and white, and characters can be antagonists without being associated with Voldemort.
However, not every Harry Potter character has this depth. In the third book we are introduced to Macnair, the Ministry of Magic executioner who’s tasked with killing Buckbeak. The books show him as a little bit too bloodthirsty, and obviously, Harry, and us the readers, dislike him because of what he’s there to do. Surely this is just narrator bias though, right?
Nope. In the next book we learn that he’s literally a Death Eater too. And in Rowling’s original outline of the fifth book, he’s the most mentioned death eater after Lucius. If he’s not a good person, even if only from the limited viewpoint of the protagonist, then clearly he must be a Death Eater and Rowling confirms this. Some characters really are that one dimensional.