Final answer:
Malcom and Donalbain flee Macbeth's castle at the end of Act II, fearing for their own safety.
Step-by-step explanation:
The BEST explanation for why Malcom and Donalbain flee Macbeth's castle at the end of Act II is that they fear they may be killed next and want to protect themselves. In Act II, Scene 3, Donalbain says, "This murderous shaft that's shot/Hath not yet lighted, and our safest way/ Is to avoid the aim." They realize that they are in danger and leaving the castle is the best way to ensure their own safety.
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