Not every Slytherin is a total git
It's a common misconception that Slytherin house is host to every dark wizard imaginable. Alright, so its founder, Salazar Slytherin, had a tendency to target Muggle-born students with a terrifying giant snake in a specially-made Chamber every now and then... And fair enough, Lord Voldemort is more or less the most evil force in wizarding history. But it's not all doom and gloom!
Take one of Hogwarts' most esteemed professors, Severus Snape, who gave his life for the greater good, all in the name of love.
How about Regulus Black, Sirius Black's brother, who famously defected from being a Death Eater in order to try and vanquish one of Lord Voldemort's Horcruxes? Nymphadora Tonks's mother Andromeda defied the pure-blood traditions of her family (which included her sister Bellatrix Lestrange) to marry a Muggle-born: a very un-Slytherin sort of thing to do. And as for wily old Horace Slughorn, he may have had some skeletons in his cupboard, but in the end shone true in the great Battle of Hogwarts.