Razano, the examples you gave are of people who might have made an impact in one way or another, but they have not affected much change. Hitler came and killed millions of peoples; communists, Jews, Gypsies, homosexuals, handicapped individuals, and brown people. Gandhi represented the potential to nonviolent resistance to colonialism. Yet, before Hitler genocides did happen and since Hitler they continued to happen: world has witnessed the fall of the atomic bombs (Little Boy and Fat Man), and AIDS which kills every day 8,000 individuals, and is 2007 it is estimated that 2.1 million people were killed by it, the wars on Iraq, many dictatorships, killing more than Hitler did in WWII. Since Gandhi the world had seen Mandela and Desmond Tutu, Mother Teresa, persons have set incredible examples of love, compassion, and leadership.
What I am saying is that a change means just that- a change, not going back, life is no longer what it was. I do not think that Hitler has made the world worse than it was before; his actions had major effects on people- creation of the State of Israel being most immediate to my life. But he was not the peek of “evil.”
I would agree with Ghassan that the question is more about a creation/invention of some sorts that changed our world forever- the US army for creating the Internet.