Question

After putting Madeline in the vault, Usher’s madness begins to increase. Why does he get so much worse?

Answer 1

Usher's madness begins to increase because even though he knows his sister is dead, he can hear the sounds of knocking and scratching coming from her coffin. She is dead, but came back to life to haunt him, and thus his madness grows stronger due to his fear of her and death. 

Answer 2

He hears the sounds of Madeline below.