The McGonagall quote is on pages 601 and 602.

It says "If you wish to leave with your students, we shall not stop you. But if any of you attempt to sabotage our resisitance or take up arms against us within this castle, then, Horace, we duel to kill."

Then during the Last Batttle so to say, it says that Voldemort was dueling McGonagall, Slughorn and Kingsley all at once! on page 735.

So, Slughorn was fighting FOR Hogwarts!!!!!!!!