Professor Minerva McGonagall
Quote 1
"I shall expect you and the Slytherins in the Great Hall in twenty minutes, also," said Professor McGonagall. "If you wish to leave with your students, we shall not stop you. But if any of you attempt to sabotage our resistance or take up arms against us within the castle, then, Horace, we duel to kill."
"Minerva!" he said, aghast.
"The time has come for Slytherin House to decide upon its loyalties," interrupted Professor McGonagall. (30.63)
Slytherin House, led by Professor Slughorn, is forced to decide whose side it's on – the school's, or its most famous alumnus's. Professor McGonagall's ultimatum makes it pretty clear that the time for moral ambiguity is past.