Complete Monster: Fenrir Greyback is just as vile as he is in canon, perhaps even more. Furious that Magical Britain revoked all rights to werewolves, he decides to wage total war on it. To do so, he leads his pack through a campaign of mass murder, killing dozens of wizards and Muggles alike. Moving north while dodging the police and Aurors, Fenrir decides to capture Harry Potter to spread his curse to him. When Percy interposes himself, Fenrir mauls him, and later brutally attacks Hogwarts in a Suicide Attack with the explicit goal of causing as much damage as possible. A sadist who gleefully admits his crimes at his trial, Fenrir gloats that he succeeded in radicalizing more werewolves by causing the laws to be further tightened, showing that he doesn't care about his fellow werewolf any more than his fellow man.
Informed Wrongness: We're supposed to agree with Harry and his friends when they concur that it would be unfair for them to buy a Firebolt to compete in Hogwarts Quidditch matches, as well as hypocritical since they all criticized Draco for getting his father to donate Nimbus 2001s to the Slytherin team the previous year. This is despite the fact that there are no restrictions on what model brooms can be used in matches; in fact, it's stated that Ron and Ginny's brooms are specifically designed to give them an advantage in their Quidditch positions compared to other models. Moreover, the reason they all disapproved of Draco's father's donation was because Draco used it to bribe his way onto the team. Harry, Ginny, and Ron, on the other hand, already earned their positions fairly through tryouts, so it makes little sense how they'd be hypocrites for choosing to buy better equipment afterwards.
Obviously Draco Malfoy was always a git, but he finally burns all his bridges in the fifth-year Quidditch match against Gryffindor, when Umbridge issues a decree that nobody is allowed to act against the Slytherin Seeker regardless of what he does, which results in even the rest of the Slytherin team focusing more on helping Draco's victims then playing the game and absolutely nobody "celebrating" when Draco catches the Snitch after breaking Ginny's nose and throwing her off her broom.
Crabbe crosses his when, right after Umbridge declares that the Inquisitorial Squad has been granted permission to use corporal punishment, he goes after a younger student without provocation and beats her so hard that she has to be immediately taken to the Hospital Wing, and he's sadistically grinning as he does it. Needless to say, this ensures that the rest of the school only escalates in their war against the Squad.
The ritual required to give Voldemort his temporary body (as shown at the beginning of Goblet of Fire) involved sacrificing a pregnant mother so that Voldemort could essentially possess her unborn child and twist that body into a form he could use.
Cedric's tortured and mutilated corpse suddenly appearing in the middle of Hogwarts - and the Triwizard Cup forming the Dark Mark above him.
Strawman Has a Point: When Luna identifies Rita as a tabloid journalist whose stories are garbage, the latter insults Luna's father for not caring about the truth and only publishing conspiracy theories and esoteric nonsense in The Quibbler. This is meant to show that Rita's a rude hypocrite. However, she isn't far off about the kind of stories found in The Quibbler and how they generally have no basis in truth. It's even stated that no one takes the magazine seriously and that people mainly read it for the entertainment value of its outlandish claims.
