"In any case," said the man, "if there is anyone who can be said to be responsible for Miss Granger's death, it is myself, not you. It is I, not you, who should have -"

"I perceive that you have spoken to Professor McGonagall and that she has given you a script to follow." The boy did not bother keeping the bitterness from his voice. "If you have something to say to me, Professor, say it without the masks."

My confidence that Quirrel did it just shot up from 90% to 99%.