During the Elizabethan Era, many believed that murder caused a disruption in natural law.  The Old Man and Ross are discussing the crazy occurrences during the night and the day following Duncan's murder.  When they speak it is daytime, but it appears dark like night.  The Old Man tells of an owl that "hawked at and killed" a falcon.  Then the men talk of how the horses went crazy and ate each other.  Ross begins with how the horses

"Turn'd wild in nature, broke their stalls, flung out,
Contending 'gainst obedience, as they would make
War with mankind."

These strange happenings directly influence the atmosphere during this scene.  The men see how nature has gone wild and has become destructive, just as someone (Macbeth) has also become wild and has savagely killed their king.  The dark atmosphere and unnatural occurrences in nature directly relate to the evil that is going on around them.  Because these things are happening in nature, the men know that they are results of something sinister going on with the king's death and Macbeth's appointment to become king.