Two conspicuous examples of chance events are connected with the murder of King Duncan. Macbeth knows that he should also kill Duncan's two sons while they are at his mercy in his castle. He may be planning to kill both of them after killing Duncan, but many unforeseen things prevent him. He tells his wife that one of the grooms cried "Murder!" in his sleep and both of them woke up briefly. This must have made Macbeth fear that the groom could have awakened guests who were sleeping in the castle. Then he tells her he thought he heard a voice crying "Sleep no more! Macbeth does murder sleep...." This was probably his imagination, but it was sufficient to make him want to retreat to his chamber rather than try to murder both of Duncan's sons sleeping in separate rooms.
There is an excellent possibility that Malcolm would have been elected king if he hadn't fled with his brother. Duncan had appointed Malcolm the Prince of Cumberland and publicly declared that as his eldest son he was heir to his throne. But the unforeseeable flight of the two brothers gives Macbeth an opportunity to blame the king's murder on them and to claim the crown by default.