Authoritarianism
In politics the abuses of both progressive and conservative political movements are due to Authoritarianism rather than Communism, Socialism, Fascism or any other any other political and economic systems that have existed. The ones that are good allow the most freedom to citizens in both social and economic terms, and the ones that are bad restrict those freedoms.
Typical right-wing arguments against Socialism use the most authoritarian counter-examples, like the USSR or Venezuela under Chavez, when there are plenty of examples of European countries that thrive under various forms of social democracy with much greater degrees of personal freedom than the US. That additional freedom (and the happiness citizens report as a result) is apparent when you consider the economic freedom that the vast majority of people gain via social safety nets, universal health care and employment regulations. These protect citizens from the ups and downs of capitalism and ensures a minimum standard of living regardless of circumstance. Freedom from destitution is desirable.
Typical left-wing arguments against Fascism of course compare all things right-wing to Hitler, even though Libertarians have been ahead of the Democratic Party on drug legalization, gay marriage, opposing the police state, and a number of other progressive social issues despite leaning right on economic policy.
In the vast majority of modern online discourse, anyone who labels someone else a Communist, Socialist, or Fascist can be substituted with Authoritarian and the result will be more politically and historically accurate and less partisan. Self-described communists and fascists notwithstanding.
We should strive to minimize support for authoritarian leaders, and to set up balance of power systems in government and the economy to limit the damage they can do.