harry potter question i need answer?

If Salazar Slytherin had it his way, it would have been but the sorting hat didn't consider those things. It looked at things like ambition and a disregard for rules. Aside from Voldemort, Snape is also half blood and a Slytherin.

Students that are place in the slytherin are interested in the dark magical art. In the book, Voldemort is one of the most power wizard because he uses dark magic to obtain his goals. It doesn't matter if a student half blood or quarter blood, the hat places students in either one of the 4 houses depending on their personality.

There is technically no rule that only purebloods can get into Slytherin, its just that most of the people there in the books have a warped idea that its better to be a pureblood. Even though the Dark Lord himself was a half blood. Ironic, isn't it?

Slytherin doesn't accept muggleborns (or mudbloods). Snape was also a Slytherin and he was a half-blood (he even lived in a muggle neighborhood). Voldemort was a Slytherin because he was the heir of Slytherin (his mother's family, the Gaunts, were the last living descendants of Slytherin) and he was sly and cunning, not to mention pure evil.