The world of Harry Potter is full of characters that are easily despised. Given how JK Rowling paints the bad guys (mostly belonging to Slytherin) without any nuance, it feels slightly juvenile to despise them beyond an “ugh, this one’s the worst!”.
Hating on Voldemort isn’t thrilling enough because he is essentially evil incarnate. It isn’t as fun shredding apart a soulless being. Dolores Umbridge, the Senior Undersecretary to the Minister for Magic and later torturer of students at Hogwarts is someone who can make your blood boil with very rational hatred. The same goes for Rita Skeeter, a stench of a journalist, and Peter Pettigrew, the cowardly sell-out.
People love to shit on Lavender Brown a lot too for simply being an annoying teenage girl in love with Ron Weasley. But, of course, in art being annoying is more of a cardinal sin than being downright evil. A better case can be built for the worrying behaviour of Cormac McLaggen, the Gryffindor jock who took to stalking and harassing Hermione. Even the pompous Percy Weasley is an easy target.
The one main character in Harry Potter, who can be deemed objectively the worst is actually the ever-parochial Vernon Dursley (portrayed with just the right amount of nastiness by the late Richard Griffiths). Harry’s aunt Petunia’s childhood rejection from her parents—who obviously preferred Lily for being the golden child of the family—and Dumbledore turned her into a scornful human. She grew up resenting her sister, her parents who favoured her so blatantly, and Harry and magic in turn because it was all just a reminder of her personal embarrassment. As much as that does not excuse an adult from discriminating against a child, there was some backstory explaining her behaviour.
Petunia’s first reaction to discovering magic was entirely normal. She was in awe and wanted to practice magic too, just like her sister. The world is full of mundane dreariness. Who wouldn’t want to be a part of a magical world if it suddenly reveals itself to us one day? Plenty of Potterheads are still waiting for their Hogwarts letters to this day, and the rest keep hoping their pet starts talking to them randomly one day.
But Vernon had no reason to be so unpleasant and narrow-minded about magic. Vernon finds out that magic is real, and he chooses to be a hateful Muggle. His loathing is largely unsubstantiated. But some of Vernon’s dislike stems from fear. He is a bully, and it is easier to torment people who are smaller than you.
Vernon’s behaviour is quite reflective of Christianity’s historic hostility towards Paganism and the occult. Plus, his character is also a stand-in for unimaginative, straight-laced (aka dull) people who tend to abhor strangeness in others simply because they do not relate to it. They like to ‘other’ people they do not understand or cannot control. Kind of like how JK Rowling tends actively to spew venom on trans people.
