Well, my real question was whether Joss means for us to hate the characters in the end - I guess I titled the thread poorly.
Oh. Well, I don't think that it's intentional then. It's just how it is, well-written characters are supposed to be hated and loved. Well-written characters are like humans, there are good things about them, there are bad things about them, things to love, things to hate. Then all those things get into the brains of the readers or the viewers, are filtered and rectified by their personalities, experiences and memories and thus an emotional response is formed. The character gives you a loaded gun and you either pull the trigger or you don't. Well, unless it's a bland character you just don't give a shit about, then you just leave them with that gun so they can hopefully put themselves out of their misery and stop polluting the work of art you are consuming. Thing is, the characters will be hated or loved regardless of author's intention because emotional response is not exactly based on reason. It's like with people.
My mother has a friend, a best friend since high-school actually, I even used to call her "aunt" when I was a kid. She's a truly good person, she's never done anything bad to me and guess what, I still hate that bitch. I can't stand her. I will never let her know how I feel about her, I will never do her wrong in any way and because of her relationship with my mom I will help her out if she ever needs my help but that's just how I feel about her. It makes no sense, it's irrational but I still boil inside whenever I see her stupid face. Bitch.
Yeah. People will love or hate a character and the author can only influence it to an extend. For example, you might hate Spike because you are a victim of a sexual assault or you might hate him just because. Similarly, I like Willow. There is a lot of reasons to hate Willow, I get that, I mean, I'm often the hate train's engine driver here but guess what, I don't care. I still like her, the more of a bitch she is the more I do. I can rationalize it, there is a lot of reasons to like Willow too after all but the truth is, the character just resonates with me. Certainly because of what the author did, but not because of one particular thing or a set of things. Point is, I don't think that it's even possible to make a well-written character universally hated. There will always be something to love about them.