Macbeth is referring to Banquo and his son Fleance, as the witches told Banquo that his children will be heir to the throne. Macbeth himself decides to get rid of Banquo and his son Fleance, who he distrusts because he believes that Banquo will get Macbeth killed so that his son could take Macbeth's place. Macbeth's own fear and conscience drives him to plot a murder against Banquo and his son. Banquo gets killed but like the witches claimed Fleance would become king, he never does become king, which proves that the witches' prophecy was invalid.
On the other hand others might believe that fate controls a person's actions. They might believe this way because Macbeth's fate was to become a king and he accomplished this goal by killing Duncan. However this argument is invalid because Macbeth's conscience and fear controlled his actions. If his fate was controlling him, Macbeth wouldn't have felt the guilt after murdering Duncan and Banquo. "Still it cried, "Sleep no more!' to all the house. Glamis hath murdered sleep and therefore Cawdor shall sleep no more. Macbeth shall sleep no more'" (Shakespeare 58). This dialogue of Macbeth explains his guilt and his fear that since he had murdered Duncan while he was sleeping, he had murdered his own sleep as well. Macbeth is hallucinating and thinks that he hears someone saying "Sleep no more." In actuality, the voice is his own conscience. However this isn't the only time that Macbeth hallucinates, when Macbeth gets Banquo killed, he is "haunted " by Banquo's ghost. "Avaunt, and quit my sight! Let the earth hide thee. Thy bones are marrowless, thy blood is cold. Thou hast no speculation in those eyes which thou dost glare with"" (Shakespeare 116). Macbeth is the only one that sees Banquo's ghost which shows that Banquo's ghost is not real but an illusion of his evil deed that he did. Anxiety and remorse have both overwhelmed him. Therefore, if fate had controlled his deeds, he wouldn't have felt a sense of remorse and such depression that he had started to hallucinate.