Shelby, you don't have to be in Slytherin to be a baddy. Look at Pettigrew, I cannot imagine him being in Slytherin becuase he was friends with the marauders. And it's mathematical as well. You've got to figure that Slytherin can only produce so many witches and wizards and when you consider the fact that not all of them turned out to be terribly evil, I.E. professor Slughorn, and you figure that Voldemort's Death Eater forces are considerably numerous you've got to figure that he got some of his wizarding warriors from other houses.

And Yes, to answer your question, Voldemort was beyond the shadow of any doubt in slytherin.