macbeth.

 I believe you are referring to Macbeth's dagger soliloquy. Macbeth is mesmerized by what he is about to do. He is acutely aware of how evil it is to kill the king. Consequently, he brings up all kinds of dark nature imagery to represent the evil deed. Macbeth often goes for dark natural imagery (to match when he first beheld the witches) to complement his tortured psyche.   

Nature seems dead, and wicked dreams abuse
The curtained sleep. Witchcraft celebrates
Pale Hecate’s offerings, and withered murder,
Alarumed by his sentinel, the wolf,
Whose howl’s his watch, thus with his stealthy pace,