The messenger arrives at Macduff's castle and issues a warning to Lady Macbeth by uttering the following:
" ... I am not to you known,
Though in your state of honour I am perfect.
I doubt some danger does approach you nearly:
If you will take a homely man's advice,
Be not found here; hence, with your little ones.
To fright you thus, methinks, I am too savage;
To do worse to you were fell cruelty,
Which is too nigh your person. Heaven preserve you!
I dare abide no longer."
The messenger arrives soon after Ross had issued a similar admonition to Lady Macduff. He then left as as quickly as he had arrived, since there was impending danger.
The dramatic purpose of this is to emphasize the degree of turmoil in Scotland at this time. It is to indicate that Macbeth's tyranny has reached such a stage of ruthlessness that he will stop at nothing to ensure that he retains his position. He has also admitted that he is so steeped in blood that there is no return. He has become so paranoid and possessed by evil, that he is left without a conscience. He has become remorseless and has stooped to such a low level that he has no qualms in killing defenseless women and children, as would happen later in this scene.
The messenger's words also juxtapose Macbeth's utter savagery to the kindness and gentleness of the messenger. The humble messenger shows care and concern for Lady Macduff and her children, whereas Macbeth's actions in sending assassins to kill them is a clear display of his callousness. It is ironic that Macbeth should be so heartless towards the family of one of his erstwhile closest friends, a man he accompanied in battle, fighting side by side for the honor of their king. This provides further proof of Macbeth's descent into paranoia and madness.