MALCOLM. This murderous shaft that's shot Hath not yet lighted, and our safest way
Is to avoid the aim. Therefore, to horse,
And let us not be dainty of leave-taking,
But shift away. There's warrant in that theft
Which steals itself when there's no mercy left.
- William Shakespeare, Macbeth, Act II, scene iii
What reason do Malcolm and Donalbain have for fleeing Scotland?

The line: where we are/There's daggers in men's smiles: the near in blood/  The nearer bloody supports the idea that Malcolm and Donalbain believe they need to flee Scotland before someone ties to murder them, too.

Explanation:

Macbeth is determined to come to the throne, and goes after anyone who stands on his way. Together with his wife, he plots king Duncan's murder.

In Act II, Scene III, Malcolm and Donalbain decide to flee Scotland. As their father, king Duncan, has been murdered, they are afraid that the same might happen to them, too. Donalbain argues that people will smile at them while hiding knives, and that they should beware their relatives, as they are most likely to try to kill them.