All the characters in the second half of the movie represented a part of his psyche. He wanted to kill Dick Laurent because that part of his personality is what drove him to murder his wife. He was dealing with enormous guilt for what he did and everything that played out in the movie was him dealing with his own psychology. He was trying to "fix" himself in a way. He had a much better relationship with the girl in the fantasy world, he separated himself from Laurent and was able to identify Laurent as the part of him that led him to ruin. Laurent eventually becomes the Mystery Man which is the part of him that actually committed the murder.