I don't think your example of evil dictators is as "cut and dry of right and wrong as you think". There are 2 sides to every story. Let me point just a few of your examples to show you what I'm talking about.
P.W. Botha, as President of South Africa during the Apartheid, he became the international "face" of the Apartheid. However, did you know he was actually responsible for creating a "thaw" in Apartheid. Prior to Botha, the South African Government was far more brutal dealing with the ANC and other political groups. Botha was responsible for repealing the inter-racial marriage laws, he relaxed many of the laws separating the races in common areas. He also created a council that encourage dialoge between the races - did it have any political "effect", of course not but considering what type of brutality existed prior to Botha, it was a massive step. Was Botha a supporter of the ANC - absolutely NOT, would he budge on all White Rule - absolutely not. However, he was the first President of South Africa to start "relaxing" what were severe "lines in the sand". Many White South Africans thought he was "too liberal" for the times. It just happened that Botha was the man on people's TV sets and picture in the paper when the issue of the Apartheid came up AND the sanction set on South Africa were mentioned in the World Press in the 1980's. The media perception can be very misleading.
You mentioned Ngo Dihn Diem. However, you didn't mention Ho Chi Minh. Ho Chi Minh was responsible for sending terrorists in Diem's country. Minh's North Vietnamese military force went into South Vietnamese Villiage and took food, supplies and men for it's army. Either a teenager would join the Viet Cong or there family would be shot in front of them. Do you think Diem's Presidency would of been as corrupt as it was if there was no Ho Chi Minh or no United States support in his country would of resulted in a much different Vietnam. The wealth that is brought into a country when a large military deployment is happening is enormous when you consider the South Vietnamese economy before the US came.
There is no question the Founding Fathers would NOT of have a problem with the United States dealing with Dictators at the International Level. Ironically, the Founding Fathers may have been educated in small schools in the colonies or at home by there Mother's BUT they were anything but "rubes". Thomas Jefferson, John Adams, John Quincy Adams and Ben Franklin had outmanevered the French, Dutch, Spanish and English all throughout the birth of the United States. Heck, the American Revolution started a World War - yet the American Army never fired a shot outside it's borders (there were some Naval Battles and the War of 1812 was fought but the US pretty much stayed home).
The Founding Fathers would of put "the good of the Country" over the philosophical difference of Government with a foriegn power. The Founding Fathers were well aware of the importance of International Trade, they knew that Trade was the best way for a Country to acquire wealth (both for the country as a whole AND the individual citizen). Self Determination and Survival was the rule of thumb for the early United States
I think today, the United States as the sole World Power has a responsibility to lead the world by example. The US has to use "common sense" when it comes to World Affairs. It's not morally correct for a country to do nothing while genocide is happening (too bad Europe doesn't share those same moral convictions - they talk the talk but they don't walk the walk). At the same time, it's not the United States place to tell another country how they can govern themselves, however we should make it no secret that we support "Democratic Based Government" and if we encourage countries to do that by opening up new trade opportunities or new business opportunities - then so be it. Being a World Power means more then just being able to "beat the other guy up" but using economic power. Listen the more the world's countries trade with each other and rely on each other for successful economies - the more peaceful the world is. The Global Economy is the World Biggest and Best Peace Treaty. It may not always be popular but it does have some important benefits.
I don't agree with you on the "Isolationism" idea that the "US wanted to stay out of Word Affairs." I think the US was quick to avoid being put into a situations it didn't want to be. The US didn't want to get mixed up in feud's between rival European Monarchies - that is precisely what the American Revolution was all about. Hey, the simple fact of where the United States is located on a map has helped it's preservation and growth. Historically, there haven't been any substancial weapons of war that would make an attack on the United States successful.
In the 1800's, there is no question that the US would want to be involved in negogiating new world trade routes. That