At the end of Act 2 Scene 3 in Macbeth, Malcolm and Donalbain decide to flee Scotland--Malcolm to England and Donalbain to Ireland. The brothers fear that the murderer is still among them, and they do not want to risk their lives by staying around. Donalbain says that there are "daggers in men's smiles," and they do not trust anyone around them. King Duncan has just proclaimed that Malcolm shall take the throne after him, and they fear that someone is out to murder the entire family. They decide to go to separate places so that if one is followed, the other will survive. Their decision is one of familial preservation.
The slain body of King Duncan has just been discovered by Macduff and the alarm was sounded. All the nobles are now gathered in the courtyard, including Malcolm and Donalbain.
Malcolm, the eldest and proclaimed heir to the throne whispers to Donalbain:
Why do we hold our tongues,
That most may claim this argument for ours?
He is asking his brother why they remain silent and allow others to speak on their behalf. It is a matter that affects them most directly and they should therefore speak up. Donalbain, in his reply, states:
What should be spoken here,
where our fate,
Hid in an auger-hole, may rush, and seize us?
Let 's away;
Our tears are not yet brew'd.
He says that they should speak about their fate (their possible deaths) which could be hiding anywhere in Macbeth's castle. Donalbain expresses fear that they might be killed next, for whoever planned their father's murder would definitely also want to be rid of them, since they would be a hurdle to whatever ambitions the murderer might aspire. He recommends that they should flee, since they have not even had an opportunity to feel sorrowful at their tragic loss.
Malcolm states that their immediate flight is more important than expressing their sorrow. The urgency of their situation cannot be ignored and they have to depart post-haste, they might be murdered if they tarry.
Malcolm requests that he and his brother not keep company with the rest of the men and he states that expressing sorrow without real feeling is a duty that some do without any true conviction or depth, it is merely an act which they commit to out of courtesy. He obviously does not want to show grief just because it is expected of him. He wants to express genuine sorrow. For a dishonest man it would be easy to do so. He tells Donalbain that he will head for England.
Donalbain declares that he will flee to Ireland. It is clear that he believes that going their separate ways will ensure their safety, for it would make it difficult for their enemies to find them at once. Donalbain clearly implies that he suspects Macbeth when he says that
There's daggers in men's smiles: the near in blood,
The nearer bloody.
Macbeth is their closest relative and he stands to benefit most if they should be out of the way. The crown would pass to him. Malcolm uses a very apt metaphor by stating that the murderous scheme has not yet reached finality. He compares it to an arrow still flying towards its target. They are that target and they therefore need to steal away quickly, leaving everyone unaware that they have departed. He says:
there's warrant in that theft
Which steals itself, when there's no mercy left.
He means that there is a greater chance of success if they should leave secretly and not make a fuss about their departure.
It is unfortunate that their actions create suspicion that they were responsible for their father's assassination and ironically allows Macbeth to claim the throne.
Malcolm and Donalbain, sons of the newly-slain king Duncan, decide to leave the country at once. Malcolm says he'll flee to England and Donalbain will go to Ireland.
In this way the brothers hope to avoid the same fate that has befallen their father, for they know that as the king's sons and heirs, they too are likely to be targets: 'This murderous shaft that's shot/hath not yet lighted; and our safest way is to avoid the aim'. Malcolm says here that they had better not increase their risk by staying in the same place where their father was murdered. He says that whoever killed the king will now be aiming for them, but the blow has not yet fallen: 'the murderous shaft ... hath not yet lighted', and they shouldn't wait until it does; the 'safest' course for them is to 'avoid the aim', to get out of the murderer's reach altogether. They also think that it's better for them to leave separately: 'Our separated fortune/shall keep us both the safer.' It will be harder for anyone to track them both down if they are not together.
The exchange between the two brothers here is brief and tense, as might be expected given the sudden, dangerous circumstances they find themselves in. They prove themselves to be quite quick-thinking and resourceful however.