Final answer:
Malcolm and Donalbain fear for their safety following Duncan's murder and flee to England and Ireland respectively to avoid being targets themselves.
Step-by-step explanation:
Malcolm and Donalbain decide to leave because they fear for their lives after the murder of their father, King Duncan. They suspect that the murderer will come after them next, as they are his sons and heirs.
Doubting the loyalty of those around them, they decide to flee to separate countries to increase their chances of survival.
Malcolm goes to England and Donalbain flees to Ireland.