A prime quote from Macbeth that shows this is in Act 1 Scene 4:
"Stars, hide your fires; let not light see my black and deep desires: the eye wink at the hand; yet let that be, which the eye fears, when it is done, to see" (emphasis added).
Macbeth's evil actions are 'black' and dark. Shakespeare uses opposites such as light and dark, to accentuate the wicked acts Macbeth is speaking of. Also, the use of stars, and fires, both of these drive away darkness and coldness (which is also often equated with evil). Stars in the night sky, contrast starkly with the black nothingness of the rest of the sky. A fire likewise brightens the night and warms the air around it. So, in hiding stars and fires Macbeth is 'snuffing out' the light, or good, in order to conceal his underhanded dealings.
Another quote from Macbeth is in Act 3 Scene 2:
"Come, seeling night, scarf up the tender eye of pitiful day; and with thy bloody and invisible hand cancel and tear to pieces that great bond which keeps me pale! Light thickens; and the crow makes wing to the rooky wood: good things of day begin to droop and drowse; while night's black agents to their preys do rouse."
In this quote like the previous one Macbeth makes comparisons with light and dark, and hence good and bad. Day is pitted against night, but going even deeper, Macbeth himself sees those few noble characteristics left inside himself, and wishes to get rid of his 'pale' color (or those qualities.) Light no longer becomes only a symbol of good, but Macbeth connects it with weakness. Day is pitiful, paleness is weak. In this way Macbeth struggles to justify himself, comparing light (or goodness) to weakness. Thus, as all light is 'thickened' or sealed away, the blackness is left and heightens the sense of evil around Macbeth.
