Explaining villainy is the most difficult task of a biographer, but Laurence Bergreen, author of Capone: The Man and His Era, has hit upon a strangely persuasive account of Scarface’s transformation from nondescript errand boy to murderous, greedy, self-aggrandizing thug. Blame sex. Capone contracted syphilis as a teen-ager, and the disease induced a dementia that plagued the gangster for the rest of his life, causing him to swing wildly from extraordinary warmth to violent megalomania. Bergreen makes a racket about the rackets at noon at the National Archives Theater, 7th & Pennsylvania Ave. NW. (202) 501-5402. (David Plotz)