Draco cast Expelliarmus. He could have, should have, attacked, but he disarmed, instead, and then attacked no further. The effort Snape has poured into this boy for the past six years has been worth it. He will not split his soul by killing. Covertly, against the efforts of Lucius, Bellatrix, and Voldemort, Snape has taught Draco non-aggression. He has fulfilled two of his three vows to Narcissa: he has watched over Draco and protected him from harm.
