In 1921, Thomas Midgley, Jr. discovered that leaded gasoline made engines knock less. A decade later, he developed dichlorodifluoromethane, or Freon, which was widely used as a refrigerant until the 1990s.
Well, it turns out that burning leaded gasoline releases massive amounts of toxic lead into the air, and that CFCs were largely responsible for the loss of the ozone layer. Thus, it's entirely possible that this man's work did more to endanger public health than anyone else in history.
Midgely died in the 1940's, which meant that he never had to live with the soul-crushing guilt of what his inventions had wrought.