Most of Shakespeare's tragic plays feature protagonists who are possessed of a tragic flaw. The flaw is not comprised of actions but of a tendency in the character's personality that leads him to make poor choices. For Hamlet, for example, his tragic flaw could be said to be procrastination. For Macbeth, it's ambition. For Richard III, it's arrogance. For Othello, it's jealousy. These tragic flaws are often at the root of the behaviors that cause the play's primary action or conflict; for example, if Richard were not so arrogant, he wouldn't alienate so many of his allies. If Othello were less jealous, he wouldn't murder Desdemona based on an accusation and fabricated evidence.
But alongside tragic flaws, each of these characters also tends to have a character who brings out the worst in them: for Hamlet, it's his mother; for Macbeth, his wife; for Othello, his manservant Iago. These characters can be seen as reflecting the dark side to the protagonist's personality, a kind of moral mirror that encourages the character's worst impulses.