Snape is the surprise element.

I just can't get past that Dumbledore trusts him even after all he's done. So I've come up with some possible reasons why Snape might treat Harry the way he does.

One possibility is that since he knows that Harry will have to face Voldemort he's intentionally tough on Harry, hoping that this will help him build a strong resistance to adversaries. Another is that he feels so guilty about what he did, that he takes it out on Harry. And another is that he thinks Harry should hate him for what he's done so he ensures that he does.

Otherwise, it just doesn't make sense that he would give Harry such a hard time.

The other possibility is that Dumbledore is wrong, but I don't think he is. He's Dumbledore. Or maybe Dumbledore wants to keep a close eye on Snape so he has him teaching at Hogwarts. But then he would have just hired Voldemort as well.