So:
Why on earth has Snape been such a [insert words here] to Harry. Shouldn't he be trying to make him a great wizard? So he can defeat Voldemort?
Snape hates Harry. That can never be denied. His attitude has the advantage of three things
1) He gets to be as nasty to James's son as he wants. Snape is not a mature man, this might be enough for him under certain circumstances.
2) Snape is that kind of teacher. He favors Slytherin. Any change will be noticed. On top of all that, he seems to be the type of teacher who has his students learn out of intimidation or an innate hatred of him and a desire to show him up. Harry is a better potions maker when Snape isn't around
3) He has a cover to maintain. He is playing both sides, so he needs to keep Draco loving him.