Snape's speech at Great Hall in Hogwarts and the duel with Professor McGonagall

Professor Snape's speech and duel with Professor McGonagall "Many of you are surely wondering why I have summoned you at this hour. It's came to my attention that earlier this evening Harry Potter was sighted in Hogsmeade. Now... should anyone, student or staff, attempt to aid Mr. Potter, they will be punished in a manner consistent with the severity of their transgression. Furthermore, any person found to have knowledge of these events, who fails to come forward, will be treated as equally guilty. No one?" "If anyone in here has any knowledge of Mr. Potter's movements this evening I invite them to step forward. Now." "It seems, despite your exhaustive defensive strategies, you still have a bit of a security problem, Headmaster. How dare you stand where [Dumbledore] stood! Tell them how it happened that night. How you looked him in the eye, a man who trusted you, and killed him."