I think we need to think about our pre-world War II knowledge to figure this out, Hitler was a leader who was not pure but came to power over the topic of purity. According to JK, Voldemort is not the half blood prince. Does this ruin the parallels?

So what clues do we find in the books to tell us?

P.S. JK says to check Chamber of Secrets because she was going to use the title for book 2..