Yes he was in slytherin - You don't have to be pure blood to be in it, that is just what Salazar Slytherin wanted in it. They have stated that there are not may purebloods left too (OOTP).

I often wondered how Voldemort could be raving on and on about killing mudbloods and how everyone should be pureblood when he ins't pureblood himself. Does that make anyone else wonder about him?

I guess in the end he is just crazy in the killing kind of way.