A follow-up question of What did Professor McGonagall know about the Order of the Phoenix?
It seems that before the Battle of Hogwarts, McGonagall faced Snape in a brief battle. Snape escapes as a coward, as McGonagall shouted. Now did she know that Snape planned this? It seems to me that she doesn't know. In the books, only Harry and Dumbledore know that Snape is a double agent.
