What is the purpose of the appearance and speech of the messenger in scene 2? Who might have sent this message? Macbeth

Its easy to infer that the messenger is someone opposed to Macbeths rule, or possibly even a soldier who discovered Macbeths plan to murder Macduffs family. The fact that he leaves and the murders show up, show that he did not llearn of the plan in time for a proper warning, and unfortunately Lady Macduff is unable to escape.