How do short story writers pull off unlikable charters?

I know it can work. Writers pull off making readers like unlikable characters like in Bullet in the Brain and A Good Man is Hard to Find and many, many more works. I think readers can like characters for the characters they are in these and other cases. But then you take a creative writing class and all of a sudden no one likes your characters, and they tell you it's a problem. How big of a problem is it? My characters aren't trying to be best friends with you. They are caught up in a story where I had them make bad decisions. Why do people say they can't relate to a character? Have they never made a bad decision? Maybe I can't relate to the reader for not being able to relate to my character.

Okay, that was somewhat of a vent. I am in the process of putting my thesis together and going over my written feedback. Some of it seems a lot harsher than I remember. But even if it's all true. How do some writers get away with creating some nasty characters that we still want to read about?

How do short story writers pull off unlikable charters?