The whole Harry Potter crew is pretty hard on Slytherin. Apparently it's the worst house to be in and if you are a Slytherin you are doomed to be bad, but here are a few reasons I think this is not true.
It is true that Voldemort came from Slytherin. Ok, yeah, he's bad. But, what about Snape! He turns out to be a good guy right?! So this is proof that NOT all Slytherin's are bad!
Secondly, I know that the belief in only teaching pure bloods is, bad, but they are like 11 when they are sorted! It's the school's fault because the way they divide students is ridiculous in the first place, if you only expose students to people who all think the same way, they are going to start to think that only what their house members think is the RIGHT way to think. So really, it's hogwart's fault this story even exists!
And lastly, it's just plain mean to stereotype Slytherins. It's the same as saying "oh! This criminal is from this town? Then I guess all people in this town are terrible". Yeah... It is. So Next time you decide all Slytherins are terrible, remember this article.