Hitler was an
authoritarian. He was evil because he murdered millions of innocent individuals and plunged the world into war by invading and attacking his neighbors.
Economically, it's a little less certain where he was, but the
political compass people put him at center-right.
Though total lack of a government can be a problem too (see Somalia for an example), an unchecked government accountable to none is an unquestionable recipe for disaster.
If you want to show how leftist authoritarianism can be a disaster, try Stalin or Mao (both who had vast numbers of people starve to death because of poor state planning). Contrast with the various mixed economies of Europe - which are not far left, but moreso than the US.
