I am going off of the theme guilt but am struggling to establish a clear thesis and point by point compariosons.
Both of the characters have a 'heart' connection: Louise Mallard has a 'heart condition' and dies as a result of her husband's unexpected return. She is said to have died as a result of 'the joy that kills'. The narrator in the 'Tell Tale Heart' is plagued by the sound of his victim's heart. I would look into the symbolic interpretations of these two events which should present as a fascinating comparison. Also, both characters cover their true feelings from those around them until they can bear the pressure no longer.
This is a tricky one, but it'll make a great essay! I agree that the theme of guilt is a good one for this essay. You can do point by point compare/contrast on the following:
1. personality of the characters
2. the source of their guilt
3. the events that occur as a result of their guilt
So your intro paragraph can focus on the idea of guilt and maybe tie it into the seven deadly sins and how guilty feelings are a result of our conscience.
Your thesis could be Mrs. Mallard and Poe's madman can best be compared and contrasted based on their personalities, the underlying sources of their guilt and the events that occur as a result of these feelings of shame.