When I first joined the University of Hawaii in 1972, one of professors in the Civil Engineering Department (which I later joined) was Rudolph Szilard. There was some connection with Leo Szilard I never was able to figure out. Rudolph emigrated to the United States from Hungary in 1949 and passed away two years ago in Colorado.
Szilard knew that Hitler's Germany had begun a project to build an Atomic Bomb, but they could not get the graphite to work, so turned to heavy water, and therefore invaded Norway, which happened to have the world monopoly of these hydrogen isotopes. You might have seen the movie, The Heroes of Telemark (Kirk Douglas, Richard Harris), where the Norwegian resistance prevented the Nazis from building this bomb.
Werner Heisenberg, who was in charge of that German project, and his team were not close to building that bomb, for his calculations were all wrong. Not quite the lie of Bush's Weapons of Mass Destruction and the Iraq War, but fortunate, indeed, for the American leadership diddled around with mistrust and indecision for several years before getting their act together. That third letter beseeched FDR not to drop the bomb on Japan, but to instead bring their representatives to Nevada to view the destructiveness of this device. However, FDR died, Harry Truman took over, and there were doubts anyway that Trinity (below, 0.16 seconds after explosion; click on You Tube recording) would even work. When it did, Truman did not hesitate to end the war by using Little Boy (over Hiroshima) and Fat Man (over Nagasaki).
Incidentally, Szilard conceived of a cobalt bomb that left nasty radioactive particles on the ground. Note that Hiroshima and Nagasaki today are thriving cities. Cobalt-60 has just the right potency and half-life (bit more than 5 years) to be ideal for eliminating life. (Do worry about this isotope from Fukushima.) The C-Bomb is the true "Doomsday Bomb" because a sufficient number of these explosions could end life as we know it! If Iran and North Korea were fashioning cobalt bombs, the situation would be very, very serious. You've got to wonder if any cobalt bombs are in a country's stockpile. One of these get stolen and you have a reason for another movie. Hmm...
Einstein said "I made one great mistake...," and that was his role in the creation of the Atomic Bomb. Szilard was even more remorseful, and completely changed his life, becoming a molecular biologist.
So is Szilard the Man Who Saved Humanity? Well, probably not. That's why the question mark in the title. In fact, he could well have created a monster, for our society was one button push away from a nuclear winter. If Hitler was actually close to perfecting the first Atomic Bomb, then, perhaps, yes. But the good guys won and the Cold War ended. No Mutual Assured Destruction anymore as the U.S. and Russia are slowly, too slowly, beginning to reduce nuclear capacity. Iran and North Korea are toast if they even attempt to use these weapons. Yet, that Cobalt Bomb still worries me.