What are Rodericks reasons for placing Madeline in the vault below the house

In Edgar Allan Poe's story "The Fall of the House of Usher", Roderick believes his sister Madelin, who suffers from a disease that puts her into a cataleptic state, has died. Therefore, with the aid of his visitor - -the narrator of the story- - he takes her to an underground vault and puts her in a coffin. However, this burial has been premature, and after a long, desperate struggle to escape the grave, Madelin makes an appearance in Roderick's room.