The problem of Slytherin is that JKR has significant authorial bias. She is a Gryffindor, this is a heroic epic, and Gryffindor qualities are favored because those are the qualities she values most in herself. Gryffindor actions are consistently downplayed - Sirius’s prank against Snape, the twins’ near-manslaughter of Montague, Hermione’s scarring of Marietta Edgecombe—have no long-term repercussions and aren’t treated with the weight they deserve (and in Hermione’s case, JKR has defended the action in interviews.) Meanwhile, Slytherin House is generally portrayed negatively: Gryffindor is treated like the hero House and Slytherin like the villain House, when the possibility exists that they could have been so much more. The world, after all, does not exist in terms of pure good and evil—and if the Houses are really meant to be as good and as bad as portrayed, well, there are the problems with world-building and plausibility that I’ve mentioned above.