I didn't realize how important Slughorn was going to be. I should have though. Rowling rarely introduces a new Character just to do so. Every once and a while someone will ask her about a character that disappeared and she'll say that she wanted to do more with the character, but it just wasn't important enough for the story.

Given that Slughorn was head of Slyterin and it looks like Voldemort was part of his collection based on the past pensieve experience, it looks like he had some hand in Voldemort's success. At least we know (or at least I think we do because he tried to cover up the evidence) that he regrets this decision.

Because Harry has avoided him and his parties I really don't know all that much about Sluggy.

The other interesting revelation was that Voldemort didn't care about his magical family either. Otherwise he wouldn't have framed his uncle for killing his father. I don't think it's that he wants to be alone. I think he just wants to be on top and will do whatever it takes.