Final answer:
Macbeth was disappointed because he realized he would need to take action for the witches' prophecy to come true.
Step-by-step explanation:
Macbeth was disappointed, especially given the witches' prophecy that Malcolm was appointed Duncan's heir, because he realized that he would have to take action in order for the prophecy to come true. Initially, Macbeth believes that the prophecy will happen on its own, but when he sees that Malcolm has been named the heir, he realizes that he will need to assassinate Duncan to gain the throne. This realization creates disappointment because Macbeth desires power, but he is not willing to take the necessary action to achieve it.