Gryffindor is known as the house of those Hogwarts students who are brave, daring, and determined - and this is often interpreted as being heroic, too. Likely because Harry, Ron, and Hermione are all Gryffindors, this house has a reputation for being the 'good guys'... but that doesn't mean that there are no villains in the common room!
While most of the Gryffindors are good-hearted and chivalric, there are some who have their villainous moments - and others who could even be painted as pure villains from start to finish. Some may argue that those characters don't belong in this house, but others will know that the traits of any Hogwarts house don't determine whether someone is a hero.
Romilda Vane Gave Harry A Love Potion
Romilda Vane is a relatively minor character in both the books and the films, but one who does something truly awful - in giving Harry a love potion in a box of chocolates. Ron, of course, ended up eating the chocolate and becoming utterly besotted with Romilda - and in the course of curing him of the effects of the potion, Professor Slughorn accidentally gave him poisoned mead, almost killing him!
Of course, Romilda has no idea that her potion would almost be the death of Ron, but it still doesn't excuse the action. While love potions are seemingly legal in the Wizarding World, they bring up serious issues of consent and control, and slipping one to a crush is absolutely the mark of a villain.
Fred & George Kept The Marauders Map
It's difficult to see Fred & George as anything less than heroic, and most of their pranks can be seen as either harmless or in the service of good (like making Umbridge's life difficult). However, their choice to keep the Maurader's Map, then give it to Harry at a point in time where a killer was on the loose and looking for him, was borderline bad-guy.
Of course, as mischievous teens, giving up such an incredible item may have seemed like too much of a sacrifice, and of course, fans know that Sirius didn't actually want to hurt Harry. However, knowing that the map included passageways that the Headmaster wasn't aware of, keeping it a secret was definitely more foolish than brave.
Hagrid Kept A Giant Spider In The School
Hagrid is another character that it is difficult to see as a villain, because he's just so lovable - and incredibly well-intentioned. However, his penchant for making friends with monsters occasionally puts him in the villainous category - most notably when raising Aragog, and then in breeding the Blast-Ended Skrewts.
In the first case, he was still a child, and can be somewhat forgiven for a decision to keep an Acromantula in the school, with the honest belief that it would never harm anyone. (An incorrect belief, as fans know thanks to Harry and Ron's adventures to meet Aragog... where they were nearly eaten!) However, a decision to secretly breed creatures that could sting and burn, and then have students tend to them, was more than questionable. It does fit the Gryffindor trait of impulsiveness, though.
Cormac McLaggen Harassed Hermione
For the most part, the reserve Keeper for the Quidditch team just seemed to be a bit of a boor, and not particularly well-liked - which doesn't make him a villain. However, his behavior at the Slug Club Christmas party definitely does. Hermione ends up taking Cormac to the event, largely just to have a date (and annoy Ron), but a scene at the party sees her struggling to get away from him, telling Harry that she was trying to 'escape' him 'under the mistletoe' and that she looked 'disheveled'.
It's clear that Cormac doesn't understand concepts of consent any more than Romilda Vane does, and that he was pushing Hermione for more of a physical interaction than she expected on her Slug Club date. It's brushed off in the books as a way of just making Cormac unlikable, but in reality, Cormac being so persistent that Hermione is disheveled and feels she has to hide from him is seriously problematic.
Sir Cadogan Let In Sirius Black
Sir Cadogan doesn't intend to be a villain, and is really just a very over-enthusiastic Gryffindor that tends to cause more problems than he solves. However, it can't be forgotten that when the whole castle was preparing for Sirus Black to appear (when everyone assumed he was a murderer), Sir Cadogan happily opened the Gryffindor common room to him!
Sir Cadogan may be forgiven for this one, given that he was simply told that someone had to have a password to enter, and Sirius had all the passwords, but that still seems a bit fishy. Sirius was on the run from Askaban, and clearly not a student or teacher, so Sir Cadogan opening the door to him out of honest unawareness that it was wrong is a stretch.
Hermione Delivered Umbridge To The Centaurs
Hermione is, of course, one of the core heroes of the series - and many would argue that her treatment of Umbridge is justified. It could be seen as heroic, and a way to get a terrible person out of the way in order to go save Sirius. However, her decision to walk Umbridge into the Forbidden Forest may take her from quick-thinking heroine dealing with a villain, to a villain herself.
By bringing her here, Hermione was potentially hoping that Grawp would actually kill her - which is extreme, even for her. However, Umbridge ends up carried off by Centaurs, and while her exact fate isn't explicit, it's assumed that she is tortured and mistreated at the very least, and fans have speculated that given the mythology of centaurs, it was far worse than that. Given that Umbridge isn't even a Death Eater, just an odious woman, this may be one of the worst things Hermione has ever done.
Percy Weasley Betrayed His Family
One of the biggest betrayals in the series in Percy's decision to completely cut off his own family in favor of the Ministry - and to spend a shocking amount of time on the wrong side of Wizarding history, too. He cuts the rest of the Weasleys (and Harry, of course) out of his life, and toes the Ministry line, which ends up seeing him working for Voldemort.
Of course, Percy isn't intentionally working for Voldemort and the Death Eaters, which is what makes this villainous act forgivable. Instead, he is just young and pompous, trying to rise in the ranks and choosing to put his career above his loyalty to his family.
Dumbledore Lied To Harry & Intended Him To Die
Fans are divided on Dumbledore. For so much of the series, he seems to be a wise and kindly man, doing his best to save the Wizarding World and defeat Voldemort - until the end, when it is revealed that he knew from the start that Harry would have to die to achieve this. Snape's shock when he asks Dumbledore if he was really keeping Harry alive "just so he can die at the proper time" is shared by fans, and no wonder.
It can be argued that Dumbledore was still a hero, and that he was doing what he thought he must in order to save everyone from a truly evil man... but there's little doubt that being willing to kill a young boy to do so was far from the act of a pure hero.
Peter Pettigrew Betrayed The Potters
If any Gryffindor can be described as a villain through and through, it is Peter Pettigrew. He became a Death Eater and one of Voldemort's most devoted servants, and he was the one who betrayed the Potters. There is almost nothing redeeming about him, and he is portrayed as a coward - the opposite of any true Gryffindor!
Of course, Peter Pettigrew is also a Hatstall, and in canon, the Sorting Hat took over 5 minutes to choose his house. It seems that the Hat chose the wrong one, as he is definitely not brave enough to be a true Gryffindor, not loyal enough to be a Hufflepuff, and not smart enough to be a Ravenclaw - his willingness to kill his friends to rise to the top, though, mark him as the worst kind of Slytherin.