At the end of Act 2, Scene 3, Malcolm and Donalbain have a brief meeting at which they both decide to flee. Malcolm says he will go to England. There he will eventually manage to raise an army of 10,000 men to help him claim the Scottish crown which is rightfully his.
What will you do? Let's not consort with them.
To show an unfelt sorrow is an office
Which the false man does easy. I'll to England.
To Ireland I. Our separated fortune
Shall keep us both the safer. Where we are,
There's daggers in men's smiles. The near in blood,
The nearer bloody.
Donalbain's observation that "There's daggers in men's smiles" is a beautiful metaphor. It says a lot about men in general in just five words. When men smile they expose their canine teeth, which can be said to resemble daggers. When a man is smiling at you, he may have an ulterior motive of one kind or another, even if he is not planning to kill you.
Shakespeare used smiles as imagery in at least two other plays. In Hamlet at the end of Act 1, Scene 5, he has the Prince say
O villain, villain, smiling, damned villain!
My tables--meet it is I set it down
That one may smile, and smile, and be a villain.
And in Act 5, Scene 1 of Julius Caesarhe has Mark Antony say to Brutus and Cassius
You showed your teeth like apes, and fawned like hounds,
And bowed like bondmen, kissing Caesar's feet,
Whilst damned Casca, like a cur, behind,
Struck Caesar on the neck. O you flatterers!
It is very convenient for Macbeth that both Malcolm and Donalbain decide to flee from Dunsinane directly after they learn of their father's murder. This makes it easy for him to blame the murder on them and to get himself elected king.
Why didn't Macbeth kill the two brothers on the night he killed Duncan? Please refer to the first link below for a discussion of that question.