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

Usher's madness begins to increase because he can hear the sound of Madeline below.

Even though his sister is buried in the basement, he can constantly hear her scratching her coffin. He was tortured by these sounds until the protagonist came to visit him so he could share his troubles with him. Usher is afraid of his 'undead' sister and of dying himself, which is why he grows crazier and crazier by the day, until he is finally killed by his own sister in the end.