1) At the risk of getting lynched for that answer, it's probably Snape. I just can't get over the way he treated his students. Or worse, some of his students. If he'd at least been equally horrible to everyone it'd be easier for me to accept that that's who he was. But as it is, his targeted bullying - since that's exactly what it is - of some of his students makes me sick. He's a grown man taking it out on eleven-year-olds, with no way for them to understand or avoid that.
And I'm saying that as someone who likes and needs the tough love approach in order to learn. I'm sure I'd have learned more from Snape than any of the other teachers at Hogwarts. But I doubt he's doing it in order to motivate or push his students. He doesn't care the least bit about Hermione's motivation, or what his actions do to Neville.
Sure, he probably didn't have the easiest childhood, but neither did Neville or Harry, you know. So I don't see how his past gives him any right to treat people he's supposed to protect and teach the way he does. I have no issues with his behaviour towards Sirius or Lupin, for that matter. They are grown men too, and they can stand their ground against him. (Not to mention that they are no saints either...) But abusing his position of power to take it out on his students, I can't find any justification for that.

And as a sidenote to explain why I find that more reprehensible than say Voldemort or Umbridge, I'm supposed to forgive Snape, while I am supposed to hate the actual "bad guys". Snape was a brave guy and he risked his life over a long period of time to make something right. But that doesn't absolve him from the need to also be an at least somewhat decent person in other areas of his life, imo.