Almost as much as Albus hated not knowing things, Albus hated other people knowing things that Albus intended to keep secret. Harry clearly knew the Prophecy. Harry knew that Tom had made horcruxes. (And Harry, unlike Albus, had known how many horcruxes were out there, and where they were, but had not told Albus—and that was galling!) Harry knew that his forehead-scar contained a horcrux, and had removed the horcrux. Sirius, at that last Order meeting, had known that Tom was living at Malfoy Manor—except that Sirius, unlike Albus, knew that Tom was already dead by then.