>>195999
angelrid is not omniscient; he is an unreliable narrator. He also said later that there wasn't a single wizard who didn't go bad in Slytherin when Slughorn, Andromeda Tonks, and Merlin were part of the Slytherin House.

He is also saying it to an eleven-year-old Harry, who had just been told by his aunt that his parents were freaks and that he was lied to when she said they died in a car crash.