by highsorcerer » Sunday 31 July 2005 7:57:52am
Slughorn modified his memory because he was ashamed of his telling Riddle about Horcuxes. NO more, no less. He thought Riddle a prize member of his club at the time (due to his talent) but later regretted telling him. Slughorn, despite being head of Slytherin, is redemned. He would renounce what he told Voldemort in a second.