The thematic subject may be guilt, but the theme is that the human heart cannot endure the burden of guilt, especially in the case of murder. The guilty must confess somehow or be consumed by his/her conscience.
Hint: Whenever you are looking for the theme in a story, ask yourself, what idea about life comes to my mind after reading this story? Or, is there some kind of lesson learned by any of the characters? Usually, the theme can be applied to more than just the story.