One of the primary hallmarks of genre fiction is its moral core. The characters and their choices may be morally gray rather than the white and black of good and evil, but the reader expects that good will eventually triumph. The good guys will gain some victory, and the darkness will be banished.
If the author fails to deliver on this promise of light over darkness, she fails a fundamental promise to the reader.
In the same way, the major character or characters must have a moral core that helps them recognize the right choices and gives them the strength to follow through, whatever the cost, to reach that triumph over darkness.
Happiness can never be gained without a struggle against the forces of darkness. The darkness may be a black-hearted villain, but its most important manifestation is within the main character who must fight her inner darkness with that moral core.
Sometimes, if the main character is an antihero or shallow chick-lit heroine, the struggle will involve a great deal of protests, whining, and foot-dragging to reach that point, but that point is reached.
Betsy, the Queen of the Vampires, in the MaryJanice Davidson series, is a perfect example of this kind of character. Shallow, shoe-absorbed, and selfish, she whines her way through each book, but her inner moral core always leads her to do the right thing in the end.
If Betsy never did the right thing, this series wouldn't have been the success it is because shallowness won't hold a reader's attention or their emotions for very long.
Sometimes, in a series, a character will change from evil to good, or good to evil, but that change must be foreshadowed in earlier choices and decisions. Bart the Bad may be up to no good through the early novels, but the reader should see that he chooses not to ambush the hero because a child is nearby. This not only adds moral complexity to Bart, but also makes his move toward the light more believable.
In the same way, a good guy's pragmatic or selfish choices will foreshadow the coming darkness.