I wouldn't say I hate her, really. I just can't stand her arrogant ignorance. If she doesn't agree with something, it's stupid. If she doesn't like something someone did, they're stupid. If someone is higher up on the scale than a peasant, they're stupid. I get why she thinks this way, and I understand where she came from, but what really bothers me about her is that her viewpoint never changes. She was rarely able to articulate satisfactory reasoning for most of the things she said, at least as far as I was concerned. She has a very naive perspective on governance and peoples' abilities to govern themselves. And the thing that irked me the most is that even when I went out of my way to make her happy, she still went out of her way to tell me that I was doing it wrong and that I needed to think of the little guy first. I just really, really didn't like her as a character. If I'd had the ability to make her understand why leaders are important to a functioning society, my perspective would have been different. As it stands, though, she's permanently stuck inside her childish worldview. That's why I didn't like her.
Edit: @Nathair Nimheil: The one that really stands out to me is in her companion quest with the lord that she kills if you talk to him for too long. If, like me, you choose dialogue options attempting to tell her that the deaths caused by the lord are as much her fault as his, she acts like you're being completely insane. She refuses to see that by being a part of an organization like Red Jenny, every action she takes within that organization is going to either directly or indirectly put someone at risk or get them killed. She takes no responsibility for her actions unless everything turns out well. Yes, the lord was a terrible person. Yes, he was wrong. Yes, he started it. But if you're going to operate within a clandestine organization devoted to fighting against the nobility, you have to be okay with the fact that you might cause some of your people to die, not just the lords and ladies you're fighting.
Obviously the lord in question here is an extreme case - he's clearly in the wrong, and he is clearly a terrible person. Some might argue that he deserves to die, and I couldn't completely disagree with them. (Personally, I took all of his property in the name of the Inquisition.) The hypocrisy here stems from the fact that Sera takes this approach unilaterally with the nobility; in her eyes, they're all the same, and they are always 100% at fault. She never accepts any of the blame for her actions, at least not that I can remember, and I think it's been made pretty clear that not every member of the nobility is a monster. If you fight someone, you have to take on a certain amount of risk. And if you do that, you can't place all of the blame on the person you're fighting.