A Harry Potter fan theory might explain how the founders of Hogwarts are responsible for Slytherin becoming the home for problem students.
Typically, in the Wizarding World of Harry Potter, villainous characters are revealed to have specific or even just thematic ties to the Slytherin House. Although there are some rare examples of Slytherins ultimately performing heroic deeds (like Severus Snape, Regulus Black or Horace Slughorn), they are typically morally compromised or selfish. As explained to Harry early in the series, there are rarely any dark wizards who don’t come from Slytherin. But that might actually be by design, as it’s possible Slytherin is a dumping ground for dangerous students.
Of the four houses at Hogwarts, Slytherin is the most problematic. While each of the houses produces some negative elements, there are almost no redeeming qualities present in most Slytherins featured in the Harry Potter series. In theory, Slytherins should attract those with clever minds and ambitious hearts, which sounds like the kind of traits that someone like Hermione Granger displays. But she’s instead assigned to Gryffindor, where she grows into a more bold and heroic figure. And it’s typically the bullish and aggressive students who end up in Slytherin.
Well, a fan theory might explain why that is, placing the blame for this development on the other founds of the school. The idea from Reddit user Burnnoticelover argues that after Salazar Slytherin passed away, the other founders (Godric Gryffindor, Helga Hufflepuff and Rowena Ravenclaw) put a hex on the Sorting Hat. Instead of just sorting students into the house they most deserve, the hat would make sure to sort problem students into Slytherin. That could explain why so many Slytherin students become aggressive and dim-witted. In essence, Slytherin became a dumping ground for the students who could, in other houses, cause problems for the rest of the student body.
The theory argues that a student’s emotional state ultimately judges if someone is a Slytherin — meaning clever tricksters like Draco Malfoy or unambitious clods like Crabbe and Goyle could find themselves in the same house. It also keeps these students from influencing other students, isolating the typically darker mindset and opinions of Slytherins and keeping them from spreading more readily through the rest of the school. That likely keeps dangerous ideals like blood purity in the Slytherin population but outside of regular conversation for other students and explains why so few members of the house introduced in the series are good people.
With nothing but negative influences around, the Slytherin students effectively worsen as they grow older — a mindset showcased by Draco Malfoy. His evolution from a preening bully to a traumatized would-be-killer likely would have been stunted if he’d grown up alongside a more diverse and less aggressive peer group. It also makes it easy for the other houses to corral and contain the more dangerous students in case of emergency — such as in Harry Potter and the Deathly Hallows, where the Slytherins got detained before the final battle to keep them from fighting alongside Voldemort. In effect, Hogwarts might be partly responsible for the Slytherin’s darker impulses, as keeping them isolated allows their darker worldviews to be reinforced and elevated.
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