Typically, Harry Potter presents Gryffindors as the good guys - the constantly brave and chivalric heroes of the story - while Slytherins get painted to be exclusively villainous, as snobbish, cunning, and rotten to the core. This is far from the truth. Not only can Gryffindors be awful on occasion, but there are many examples of Slytherins being heroes.
While there are a great many awful people to come out of Slytherin House, some of the most important heroes in all of Harry Potter and Fantastic Beasts have also come out of the Hogwarts House to help save the day - both deliberately and accidentally.
Slughorn Giving Harry The Memory
Professor Horace Slughorn was introduced in The Half-Blood Prince as a potions master with an affinity for skilled students, valuable collector's items and a deep secret Dumbledore greatly desired.
That turned out to be a conversation with young Tom Riddle, who asked Slughorn about Horcruxes. Slughorn, ashamed of what he revealed to the would-be Voldemort, altered the memory, adding an interesting bit of backstory to the Hogwarts professor. Eventually, though, he gave it to Harry and, in turn, Dumbledore, allowing the pair to set off on a quest to destroy the Dark Lord once and for all.
Crabbe Accidentally Destroying The Diadem Of Rowena Ravenclaw
Vincent Crabbe is no hero. In fact, he is a low-key villain in the Harry Potter franchise and in his final appearance, he pretty stupidly attempted to murder Ron, Harry, Hermione, and Draco with Fiendfyre resulting in his demise in the Room of Requirement.
While the heroes, along with Draco and Goyle, escaped, Crabbe also destroyed the diadem of Rowena Ravenclaw, a Horcrux. This may have been an accidental moment of heroics, but it still played a part in the death of Voldemort and even continued the redemption arc of Draco.
Snape Saving Harry In The Philosopher's Stone
Severus Snape is one of the Harry Potter franchise's most complex characters, and for the longest time, fans do not know what side he is on. Nor does Harry, who believes Snape cursed him during a Quidditch match in The Philosopher's Stone.
As it got revealed at the end of the film, though, it was actually Professor Quirrel jinxing Harry. Snape, meanwhile, was counter-cursing and saving Harry. It was not a moment of saving the world or defeating the Dark Lord, but it was a smaller moment of heroics that saved Harry's life.
Phineas Black Rebelling Against His Father
It is not 100% confirmed but universally assumed that Phineas Black, son of Phineas Nigellus Black, got sorted into Slytherin house, making the betrayal of his abhorrent father all the more shameful to the Black family.
His father was the least popular Headmaster in the history of Hogwarts and a believer in blood purity who looked down on Muggles. It was a small act, but a meaningful one, when Phineas became a vocal supporter of Muggle rights, disowning the idea of blood purity and getting disowned by his family in the process.
Regulus Black Stealing The Locket
Regulus Arcturus Black is one of the most well-known and vital members of the Noble House Black in the Potter films despite never being seen in them. A former Death Eater who defected, Regulus attempted to destroy one of Voldemort's Horcruxes.
After finding out how Voldemort would bring his goals to fruition, Regulus defected and stole the locket of Salazar Slytherin, a Horcrux, replacing it with his own and asking Kreacher to destroy it. Of course, Kreacher was unable to, and Regulus got killed for his insolence. Still, just because he did not complete his missions does not mean he was not a hero for his attempts, especially since it eventually got destroyed by the golden trio.
Leta Lestrange's Sacrifice
Fantastic Beasts: The Crimes Of Grindelwald is a fairly hated entry to the Potter franchise by Potterheads, but it did give fans a lot of lore to digest, including the introduction in the flesh of Leta Lestrange.
Leta was far from what is perceived to be a typical Slytherin, and her death proves so. She pretty much gave her life to save the lives of Newt, Theseus, Tina, Yusuf, Nagini, and Jacob, as well as to destroy Grindelwald's important skull hookah. Of course, death in Harry Potter is not always the end; there may be more to Leta's story yet.
Slughorn Fighting In The Battle Of Hogwarts
Horace Slughorn is hardly the most likable individual. He may not be overly ambitious or evil, but he is dismissive and snobbish of those he sees as unworthy of his time. That does not mean he is any less of a hero for fighting in the Battle of Hogwarts.
Anyone, from students to professors to former students who fought in the Battle of Hogwarts against the forces of Voldemort, is unquestionably a hero to some degree, no matter how they are personality-wise. The former Head of Slytherin and potions master is no different.
Narcissa Malfoy Lying To Voldemort
Narcissa Malfoy may have been a Slytherin, and her family may have been loyal to Voldemort for a long time, but she cared for nothing anywhere near the amount she cared for her son. As a mother, Narcissa was fiercely loyal.
Unafraid to betray the Dark Lord if it meant a peaceful or happy life, or even just a life together away from war, Narcissa neglected to tell Voldemort Harry was alive in the Forbidden Forest. Instead, she asked Harry if Draco was alive. Since Harry was the only one who could defeat Voldemort, when he told Narcissa her son was alive, she saw nothing else but a way out for her family. Had Narcissa told Voldemort the truth, Harry would have died for real. While it was done out of care for Draco, out of selfishness even, rather than care for Harry or the fight for good, it was still brave and heroic.
Draco Malfoy Raising His Son To Be Different
Thanks to Narcissa, the Malfoy's were granted a reprieve and did not end up in Azkaban, allowing Draco to have a son of his own named Scorpius with his wife, Astoria.
While Scorpius grew up in Malfoy Manor and did have a troubled and isolated childhood, he did not get raised with the same ideologies and deplorable views on the world that Draco was. Choosing to buck the trend, Malfoy decided that future generations of his family, starting with Scorpius, would not be raised with pure-blood beliefs. Non-pure-bloods and pure-bloods were equal. Not heroic as in sacrificial or in battle, but valiant in attempts to stop the continuation of discrimination and hurtful beliefs.
Snape Being A Double-Agent
There are many mixed opinions about Severus Snape within the Harry Potter fandom. Some do not see him as a total hero, some see him as misunderstood, and some see him as creepy and abusive. In the end, though, Snape did a lot to defeat Voldemort.
Snape acted as Dumbledore's double agent, gathering intelligence on the Dark Lord and of Death Eaters. The Head of Slytherin also watched over Harry and protected him for many years, and in the end, finally revealed the truth to him. Everything he did was to defeat Voldemort and preserve the memory of Lily, and it pained him to do a lot of it, not least of all the murder of Dumbledore, which was crucial for the greater cause.