The dog ate my homework. The devil made me do it. She forced me to eat that apple.
People have been coming up with excuses for their actions since Ugg first had to apologize for hitting Zog with a rock. (The saber-toothed tiger made me do it?) And the favorite excuse of great tragedy is almost always "fate." But Macbeth questions that excuse. Is it Macbeth's fate to be a traitor and a king-killer? Or is he alone responsible for his actions, and did he freely choose his choice? The play pits the prophecies of the three weird sisters against its own dramatization of Macbeth's internal conflict—and it's not clear which wins. In fact, fate and free will might just be working together.
Macbeth leaves us hanging. It never answers the question of whether free will or fate determines a person's future.
Macbeth may be fated to be king, but he decides all on his own that he will murder Duncan in order to obtain the crown. His actions suggest that fate may be predetermined, but free will determines how a people reach their destinies.