Woodrow Wilson, whose failures of commission probably had the most dire consequences of any U.S. president. His great flaw was his sanctimonious nature, more stark and distilled than that of any other president, even John Quincy Adams (who was no piker in the sanctimony department). He thought he always knew best, because he thought he knew more than anybody else. Combine that with a powerful humanitarian sensibility, and you get a president who wants to change the world for the betterment of mankind.