The man in the last chamber is Professor Quirrell. Harry and Hermione are shocked because he is the last person they expect to see. They find out that Quirrell is not the mild-mannered teacher they thought he was. It was he, not Snape, who tried to knock Harry off the broom during his first Quidditch match to kill him. Snape was mumbling because he was setting a counter curse to stop the jinx, but was not sure who was attacking Harry. The talk in the woods was interpreted incorrectly by Harry. Snape suspected Quirrell was up to something and was prodding him to see how much he knew about the Stone and warned him about his odd actions.
Quirrell tells Harry that Snape does hate Harry; and an old vendetta against Harry's father is.....
