The developed, post-industrial nations of the world "require" resources from the rest of the world--natural resources, labor resources, and strategic resources. Most in recent history have espoused a moral standing over international affairs. For instance, the USSR claimed it supported humanistic, worker centric egalitarianism, and the USA claimed it supported democratic, human rights focused reforms.
Despite these claims, most, if not all, of the "great powers" follow what is known as realpolitik, a German political science term that means a view of international politics emphasizing national self-interest. In other words, all nations inherently act in ways to increase their own power, and therefore, all nations must act in this way, lest a competitor take power and opportunity for itself at the expense of others.
Nations do not view their actions as either moral or immoral, but rather "amoral," or morality simply does not apply. In the examples provided in the question, it is not so much that the United States views its policy as supporting ruthless dictators and regimes in South America or the Middle East, but rather that it supports whatever situation will supposedly provide easiest access to resources the United States wants.
The pendulum swings the other way, as well. For instance, post-war Germany and Japan received much support from the United States in forming new regimes and rebuilding, and now, both live up to high standards of human rights and democracy. Still, this is not completely out of high ethical standards, but rather as a way to create bulwarks against Soviet and Chinese ascendancy.
The spectrum of governments that have received or are receiving US support covers the entire range from the best to the worst. In nearly all cases, the things that tie them all together are political, material, and strategic gain for the United States. Unfortunately, this is the truth behind virtually every "great power" in history. It is not a question only the United States has to ponder, but, indeed, the entire community of nations.