Warning - Harry Potter spoilers/overly emotional rants about fictional characters ahead.
I don't care if Snape (professor Snape) works for the Order or not. He's just a jerk. I don't have a problem with him giving Harry detentions or taking points for his misbehavior. McGonagal does it too - she's very strict but at least she's fair. I'm not even that upset about what he did at the end of book 6. Whatever his motives for doing what he did - it was man to man.
The thing I hate is the petty way he picks on Harry - from the very first day they met. I hate the way he ascribes intention and motive to Harry's actions. It's so unfair for him to assume he knows why Harry behaves the way he does. And the fact that this grown man is holding a grudge against a young boy simply because he hated his father - that's just so immature. I mean get over it already! I can hold a grudge with the best (worst) of them... but even I know when to give it a rest.
I hope he kicks it in the last book. :P