Deathly Hallows?

Did anyone else notice that when the battle at Hogwarts begins Horace Slughorn decides to be a good guy but then on page 734 it says that Charlie Weasley is overtaking Slughorn and then on the following page it say that Voldemort was "deuling McGonagall, Slughorn and Kingsley all at once" What's going on? Whose side is he on?