We allied with some evil people, I'm not denying that. We have also supported terrorists. We helped found Al Qaeda, we supported terrorists in Nicaragua and Yugoslavia (and I'm sure a whole lot of other areas. Look up operation Gladio and the strategy of tension, done with CIA help).
http://en.wikipedia.org/wiki/Strategy_of_tensionWe have done evil things and we need transparency and the rule of law to investigate, prosecute and prevent these things.
Having said that, I do not think anyone but the mindless automatons on the right are oblivious to this fact that the US has both helped and stopped dictators in our history. As far as your list, the US helped bring down some of those dictators. Carter helped bring down Somoza. FDR helped bring down Hitler. Reagan sent some guy whose name I can't remember to Chile to encourage Pinochet to hold a plebicite that he lost in 1990. We donated millions to help Cambodia rebuild after Pol Pot, and Pol Pot only won after US troops pulled out of Indochina. We arrested Noriega. We invaded Haiti in the 90s to uphold the democracy. Etc.
The US has both helped and stopped dictators. But I think lately, especially since the end of the war on communism, we have moved more into the helping people with democracy. Sadly sometimes dictators are all you get to work with. Either side with the dictators in North vietnam, or the dictators in south vietnam.