At the end of Chapter IV, how does the described change in the environment correspond to Dunstan's state of mind?

Dunstan's mind is dark, filled with greed and deceit. The environment too is dark and forlorn. When he sees that Marner is not at home, Dunstan thinks, Why borrow Marner's money when he could just take it? Dunstan finds the loose brick beside Marner's loom and removes the two leather bags filled with Marner's guineas. After replacing the brick, Dunstan rushes out of Marner's cottage and steps out into the darkness of the night, carrying one of Marner's bags in each hand and still managing, with great difficulty, to brandish his brother's whip.