Harry tensed in his chair but spoke up.  "Yes, sir.  However, he was a double agent.  We believed him to be responsible for killing Headmaster Dumbledore last year, but I have since discovered that he and Dumbledore had decided that Snape should kill him in advance due to a separate plot by Voldemort himself.  Snape, despite all his issues and the mark on his forearm, was fighting for the Light the whole time."