Snape deflects McGonagall’s attack and uses it to take down Alecto and Amycus. Snape knew that he needed to get rid of them before being driven out of the castle so that they wouldn’t harm any of the students.
2001 - Alan Rickman as Professor Severus in "Harry Potter and the Sorcerer's Stone." This teeny-tiny clip is from Harry's potions class ever, and Prof. Snape had asked Harry 3 very difficult potion-related questions . which Harry couldn't answer.