Final answer:
Malcolm and Donalbain flee Scotland after King Duncan's murder because they fear they are in danger and could be framed or killed next. They quickly realize that the safest course of action is to escape the reach of the unknown murderer.
Step-by-step explanation:
After King Duncan's murder becomes known, Malcolm and Donalbain flee Scotland because they fear for their own safety. Being the king's sons, they are the immediate suspects, and staying may very well mean that they would be next in line to be killed. This action raises suspicion among the others, but Malcolm and Donalbain understand that whoever killed the king would likely not hesitate to kill them as well.
Moreover, their flight allows them to escape from a situation where they have no allies and where the truth of their innocence might not be enough to protect them.
Donalbain suggests that their tears are ‘not yet brew'd’, meaning they are not safe enough even to stop and grieve their father's death, and Malcolm agrees by stating that the 'murderous shaft that's shot hath not yet lighted', implying that the danger of the assassination has not passed and is likely to continue.