The narrator hears a scream (possibly the sound of a soul loss, a death) - and brushes off the idea that it was the sound of his own soul dying. He leaves his space of comfort and decides to investigate his motives, actions, feelings. The consequences of this and the path in front of him are the "bad thought" that stings and starts to ebb the life from his body ("cut my wrist"). He crosses the threshold to a dark world as he ventures into the darkness of his soul. ("Head for the door.")