The guilt was the reason Dorothy decided to kill all her friends. She was guilty for what she had done so many years ago. The guilt drove her crazy. He was the one guy who seemed to be nice to her and she had done that to him. It was sickening.
She had hurt him. She had made his life miserable. She had distroyed him. She never found out what happened to him, only that he disappeared. She decided to kill everyone who had turned him down. Kill everyone who had ruined his life.
She thought it would take her guilt away and for awhile it did. She started getting enjoyment from it. The police would never think it was her. She became obessed with it. She enjoyed it. She knew it would be the death of her. She no longer cared.