Your main characters need to interest the viewers. Do they have to be 'good'? No. You do need to write characters that the audience cares about. You do, however, need a good character arc that brings some 'redemption' or positive out of the character. Most people do not understand how a battered partner can stay in an abusive relationship. By analogy, how can an audience remain interested in a film where the main characters are continually abusive? Lots of people in this situation feel 'it will get better' and it doesn't. For a film audience, that leaves lots of disgruntled viewers at the end.
The lead character in "Despicable Me" is mean but transforms. The Grinch transforms. If good people can go bad (Anikin Skywalker), it can also work in reverse. If your character is unlikeable throughout, there really is no thrill at the end. The audience needs to feel some attachment.
If the protagonist is a nasty character, you need to have a strong positive antagonist. People like to think they would act positively so they often associate more strongly with the positive characters. Hannibal Lector is riveting because he is so charming and can show positive acts despite his more heinous side.
Think about all the memorable characters, what were their roles? their personalities? What often makes them interesting is they are not flat, not solely good or bad. It's hard to judge without seeing the script but you may simply need to more fully develop the characters' backstories.
