Why the Govt. of America supports anti-democratic countries/Governments even dictatorships of the world.?

American Government has been supporting economically, politically and many ways the countries having anti-democratic governments even dictatorships since long, for example, Saudi Arabia and Arab sheaiks countries, Iran when it was a Imerialist country of Shah Iran, the dictors of Cobodia, Chilli, Panama, Granada, Geotamala, Iraq when Sadam got a lot of American Aid, Talaban when they fought with USSR even though they believe on dictatorship and many countries of Asia, Africa and South America. Is it fare to support a dictator who killed millions of annocent peoples?