Voldemort was not a pure-blood. He was a half-blood. His father was a muggle. The same could be said of Severus Snape, another half-blood. Still, he was prejudiced, or so it seemed and relished in killing muggles, Muggle-born wizards (that the blood-obsessed called Mudbloods), and anyone who seemed to have a soft spot for them. It's also worth noting that Slughorn, as well as Andromeda Tonks were Slytherins, but they were not obsessed with blood purity. The more important atrributes for a Slytherin are ambition, cunning, and achievement.