Initially, when Tarquin is planning out his heinous act, he says (possibly out of guilt):
“Fair torch, burn out thy light, and lend it not/ To darken her whose light excelleth thine” (190-1)
Here, Tarquin describes Lucrece’s virtue as a light brighter than a torch (or in this case his lust). In the same stanza, he recognizes that his act is that of darkness/ evil. Tarquin calls his act the “blackest sin” (334).
This theme is also evident when, during the night, Tarquin first rests his eyes on Lucrece, he is (once again) astonished by “a greater light/… that she reflects so brightly” (375-6).
Moreover, when Tarquin initiates his crime, it is noted that Lucrece loses her ‘light’:
“… her locked up eyes,/…/ are by his flaming torch dimmed and controlled.” (446-8)
Furthermore, after Tarquin ravishes her and runs off, Lucrece expresses her shame by cursing the Night and wishing that it will never be daylight again.
Shakespeare uses the theme of light and darkness to highlight the aspects of innocence and evil throughout the play, where the light is symbolized to be pure and untouched and the darkness is evil and full of evil opportunities.