I think it's ok to hate the character, as long as it's a believable character and we hate them the way we'd hate a person in real life. If we hate them because they are too flat, or they say things that don't fit with their personality, then we will stop reading. But if we hate them because we believe they are doing/saying horrible things, and we believe they really would say/do these things, then we, as readers, are eager to keep reading to see what they will say/do next. Does that make sense?
I used the example of Touching Spirit Bear by Ben Mikaelsen. His main character, Cole, is despicable, but we believe he would be that way because of his home life. The motivation makes sense with his personality. Still, the reader Hates him until he changes about midway through the book. The reader doesn't really sympathize with him until he begins to realize how awful he's been.
In my MG novel, the main character says and does horrible things because she is so angry and sad and guilty over the loss of her sister. I think, I hope, I've painted her in a realistic fashion and that readers will stick around to see if she can get past all these emotions. In fact, my advisor read a particular scene and said she wanted to slug my character. I think that's good, because it means she believes in this character. I think any time we can get the reader to feel a strong emotion about our characters because of who they are or what they've done in the story, we have accomplished our goal of capturing the reader's attention.
Do you have any unlikable characters in your stories?