How do people who are dying treat Father Hooper? Why do they act this way?

The dying request his presence, all while shuddering in fear at his appearance.

Dying sinners cried aloud for Mr. Hooper and would not yield their breath till he appeared, though ever, as he stooped to whisper consolation, they shuddered at the veiled face so near their own. Such were the terrors of the black veil even when Death had bared his visage.