I think Snape hates Harry simply because of his father. In Harry he sees only everything that he hated in James. So now hes the one who is in control and takes it to his advantage.

And just add to a previos post about how Dubledore and Snape are working together, I would just like to add that I personally think that Dumbledore is gone for good and that Snape is with Voldermort.