The Harry Potter series may have been initially marketed towards children, but it was full of very dark villains — especially as the books and movies wore on and became darker as Harry grew older. Some of the villains were relatively harmless school rivals, irritating and arrogant but far from life-threatening. Some of them were murderous dark wizards, known for their penchant for torture and causing pain and misery wherever they went.
Although good ultimately prevailed at the end of the series and Harry went on to live a full life and have a family, some of the villains never got the consequences they actually deserved. Not always just their death, but sometimes before that, when other actions that deserved some harsh consequences saw them come out unscathed instead.
Draco Malfoy
Draco Malfoy went through something of a redemption arc, emerging from the series as someone the fans could actually sympathize with. But in school, he did numerous terrible things that he completely got away with. In one memorable scene, he paralyzed Harry and stamped on his face, breaking his nose.
Yes, he saw the light in the end, but considering he did things like this and never got punished... There were points in the series where he should have been made to learn a harsh lesson.
Lucius Malfoy
Even worse was Draco Malfoy's father, a selfish man who dedicated himself to Voldemort unless it didn't benefit him anymore. The Malfoys came out of the war very fortunate, since Harry was willing to defend them when on trial — but did Lucius Malfoy deserve to? Narcissa saved Harry, Draco came around in the end, but there's no evidence Lucius ever saw the error of his ways.
Dolores Umbridge
Dolores Umbridge almost met a harsh end when she was carried off by centaurs and almost killed by them, but she was saved in the end. Although it's unclear what happened to her, she recovered pretty quickly and was seen again in Deathly Hallows, evil as ever. This woman was one of the most atrocious in the series, abusing her power to torture actual children.
Bellatrix Lestrange
Bellatrix Lestrange was killed by Molly Weasley, so it could be argued that she got exactly what she deserved. But considering the amount of people she took great pleasure in torturing, a quick death was actually pretty merciful. She at least deserved to be made to see what she had done before she died, face the amount of the families she tore apart.
The Dursleys
In comparison to many other characters in the series, the Dursleys were not the worst. They were, at least, just privileged and selfish — not downright evil.
Still, they got lucky, able to go into hiding and be protected and return to their home when the war was over. Considering Harry went out of his way to keep them safe and give them a place to go and hide after they neglected and abused him for his entire childhood, they definitely got better than they deserved.
Severus Snape
Severus Snape, in the very end, met a pretty gruesome death. Some fans think he deserved that, some didn't.
However, he did not get the consequences he deserved when he was abusing children in school. He constantly bullied his students and for some reason, Dumbledore never seemed to discipline him for it. He let him favor Slytherin students and treat the ones he didn't like in a completely unacceptable manner, and this man should have been fired long before the end of the series regardless of the good he was doing for the Order.
Cornelius Fudge
Cornelius Fudge faded into the background with no real consequences for some very damaging actions. His denial that Voldemort had returned was severely damaging to the Wizarding World, leaving them unable to form a proper defense against him. He was a terrible Minister for Magic.
Romilda Vane
Romilda Vane was not technically one of the worst villains of the series, but fans never found out if there were consequences for her trying to slip Harry a love potion. Considering this would make someone act against their own free will and actually, was the exact circumstance in which the monstrous personality of Lord Voldemort was created, it would be a horrific thing to do and she deserved to be expelled for this.
Walden Macnair
Walden Macnair was still alive in the 2010s, giving interviews to the Auror department.
He may have been imprisoned in Azkaban for years but he deserved to meet the same end as Voldemort, Bellatrix, and some of the other Death Eaters. He was a cruel, vicious man, and fans still hold a grudge for Buckbeak.
Voldemort
Voldemort may have died in the end, but he went many years with no consequences for his actions. He was the worst person in the series, someone who murdered, blackmailed, tortured, and tore so many families apart. His death was quick and painless — in the movies, he just sort of crumbled into dust. It seems it would have been more fitting for him to have to acknowledge everything he did somehow.