The Fall of Nelson Van Alden 

Van Alden is a dark man. As a religious zealot and prohibition agent, we see him violate moral and legal concepts as a means to what he sees as right.
The end justifies the means in his mind, even it involves inflicting pain to a wounded thief and drowning his corrupt partner in a mock baptism.
Then he meets Margaret and we see him admit to his sin of lusting after her and punishing himself; he can't stop though. We further see him collapse entirely, losing all vestiges of religion and righteousness, and completely give in to every vice which he's fought against.
So he descends into the darkness.