If you want to read the most terrifying, incredibly likely theory about Harry Potter and horcruxes go NOW to SimplyKimberly and read the locked post on the matter.
Basically, what Kimberly is saying is that Harry IS one of the horcruxes. When Voldemort killed James and Lilly, he made Harry a horcrux. That would certainly explain the “traits” Harry shares with Voldemort and it sets up the gloomiest of futures for Harry. I’m so enthralled with this theory that I will support it as truth until Book 7 comes out. I think it will be proven, I really do.