In act 3, scene 3 of Macbeth, Banquo's death literally represents a loss of innocence; symbolically, the murderers' torch is extinguished, and the darkness is symbolic of a loss of innocence.
Macbeth and Banquo begin the play in a similar place. They are both loyal to their king and both hear the witches' prophecies about their futures. While Macbeth allows the witches' predictions to steer him toward increasingly evil plans, Banquo remains principled and is not involved in Macbeth's murderous schemes.
In this scene, Banquo is ambushed by murderers who have been hired by Macbeth. He recognizes the "treachery" in their actions, yet as he dies, he urges his son Fleance to escape. While Macbeth murders Duncan in an ambitious quest, his decision to hire assassins to murder Banquo reflects a deeper loss of innocence. Banquo presents no immediate threat to Macbeth, and the witches even proclaim that Banquo himself will never be king. Macbeth kills Banquo in a desperate attempt to interpret the witches' prophesies in a way that secures his own kingship, which demonstrates that his conscience is now devoid of innocence.
Symbolically, the torch the murderers carry is extinguished just after Banquo's death. Light is often symbolic of truth and goodness; the darkness symbolizes a loss of innocence and integrity.