Being a pure-blood is one of many qualities that might help someone into Slytherin. I am sure Tom M. Riddle wasn't the first half-blood nor was he the last to be placed into Slytherin. Remember Tom was the direct heir of Salazar Slytherin. He was cunning and power hungry, both qualities favored by Salazar. He would have heard the Sorting Hat's whispers just like Harry did and chances are just as Harry rejected Slytherin, Tom selected it. He may have been whispering "not Gryffindor" or "Let it be Slytherin" like Harry thought of "not Slytherin".
Dumbledore in one of his conservations with Harry, I think it's at the end of Chamber of Secrets, talks about the difference between Tom and he are the choices they had made. While the Sorting Hat had stuggled to place Harry, it finally placed him in Gryffindor because Harry chose it. |