As a writing instructor, I’ve noticed that students usually have an easier time creating “likeable” characters than unlikeable ones, which are sometimes glossed over and written superficially. Here’s a simple exercise I’ve used in the classroom to help emerging writers give unsavoury characters more depth and humanity:
Choose a scene from a story-in-progress in which you think you have given short shrift to a disagreeable character. Rewrite that scene so that the character continues to hold our attention but also arouses our sympathy or understanding, at least to some degree.