JK Rowling whilst writing the HP-series:
Slytherin is the "evil house". The guy who bullies Harry, Ron and Hermione goes there with all his mean friends. Slytherin and Gryffindor, the house I put ninety percent of my main characters are always competing. Also lord Voldemort was a Slytherin. They only care about being from a rich and well-known family. Their password to get in was literally "pureblood". Only REALLY fancy/elite kids go there. Pretty much every death eather was a Slytherin. Their head teacher is the scary-scary Snape. Wow. Such evil
JK Rowling after finishing the series:
WHO TF TOLD YOU SLYTHERIN WAS THE "EVIL HOUSE"??????? NO NO NO THEY ARE NOT ALL HOUSES ARE EQUALLY GOOD!!!!!11!!1!1!