There are few characters in the Harry Potter universe who are hated more than Dolores Umbridge. There are certain characters who are more evil, like Voldemort with his desire to wipe out the muggle race, and his band of death eaters who perform terrible torturous acts at his bidding.
There’s also the Malfoys including the sniveling Lucius who likes to portray himself as ignoble and above reprieve but is actually only a coward, and Bellatrix Lestrange, who takes joy and delight in causing the suffering of others. But none of these characters are as purely disliked among fans of the franchise as her, for she likes to sugarcoat her evil actions in the guise of heroism and justice. In many ways, this makes Umbridge scarier than Voldemort.
Umbridge first comes to Hogwarts in Order of the Phoenix, during Harry Potter’s fifth year of wizarding school. She is sent at the behest of the Ministry of Magic, to be the new Defense Against the Dark Arts teacher, after the school’s not-so-stellar track record with the post. Harry’s previous 4 tutors each had their own duplicitous sides: Quirrel turned out to be concealing Voldemort, Lockhart tried to erase his memory, Lupin was denied access to his medication and turned into a werewolf, nearly killing his students in the process, and Moody turned out to be the imposter Barty Crouch junior.
All of this combines to give Dolores every authority to undermine Dumbledore, and to eventually even take over his position as headmaster. And all of this happens at school, which is Harry’s only safe place in a world that is at war after Voldemort’s return. But when the war is won, what happens to Dolores?
Essentially, her unspeakably cruel acts get her locked in a life sentence at Azkaban. During Dolores’ time at the school, she bans all practical defense lessons, purposefully leaving students unable to defend themselves. She inflicts cruel punishments, like making Harry write ‘I will not tell lies’ in a Blood Quill that scars him for life by taking the blood from his hand to use as ink. She uses Veritose serum on students without their knowledge or consent, in order to extract information that would not otherwise be freely given, and when she fails to acquire the information she seeks, she threatens to use the cruciatus curse to torture it out of them. Her pettiness and vile cruelty stretch beyond her role, but she proclaims that everything she does is justice under the Ministry’s new regime. She aims to eradicate anyone who challenges her authority, all on the pretense of doing ‘what’s right’ for the wizarding world.
She does get a comeuppance of sorts at the clever hands of Hermione who tricks Umbridge into the forbidden forest late at night. Umbridge has an extreme dislike for ‘half-breeds’, including merpeople, werewolves, and centaurs, so when she insults and attacks one of them in the woods, she is dragged away by the herd and subjected to weeks of abject terror. Eventaully Albus Dumbledore locates her and rescues her, at which point she is returned to and reinstated at the ministry, but she is said to retain a huge phobia of centaurs ever after.
Despite her punishment for cruelty at Hogwarts, her acts of small violence at the school are nothing compared to her anti-muggle crimes in the Ministry. She sets up and runs the ‘Muggle-born Registration Commission’ in which she sentences all who aren’t considered to be from ‘pure’ bloodlines, aka those who cannot prove that at least one (but preferably both) of their parents were of magical lineage. This is what makes her a truly evil villain, and a character disliked by so many, because she persecutes innocent people, people like herself even, and calls it justice under the law.
Umbridge sentences hundreds of innocent people to terrible fates for their ‘unclean’ family history, which is made worse by the fact that her own mother was muggle, a fact that she has conveniently erased from people’s memory. She is able to wield terrible power both because she is in league with Voldemort and his Death Eaters at the time, but also because she has the influence of the Slytherin Locket, which is one of Voldemort's horcruxes and contains his evil malice.
After the horcruxes are destroyed, and Voldemort is finally defeated by Harry and his allies, the Ministry of Magic is retaken and reformed. At this time, Dolores Umbridge is put on trial and quickly and easily found guilty of the awful crimes she committed against muggles and muggle-born, as well as in aiding the Death Eaters. She is thereafter sentenced to life imprisonment in the dark halls of Azkaban prison.