Adolph Hitler
He started the largest war the world has ever and hopefully will ever see. That war killed over 56 million people including over 21 million Russians and 11 million Chinese. This figure doesn't even include the estimated 6 million Jews killed as part of Hitler's final solution.
For better or worse, World War II caused an explosion of technological advances including jet aircraft that now fly us all over the world and rockets that have taken us to the moon and allow us to explore the other planets in our solar system. These and other inventions will probably have been invented anyway but the war effort accelerated their development.
After World War II the United Nations was formed to help us resolve the conficts that started two world wars in the first half of our century. The United Nations has succeeded where the League of Nations failed.
World War II also sparked the Cold War between the U.S. and the U.S.S.R. which, while not beneficial to the world, did have a major influence for decades and still does as we clean up some of the messes left over from that era (Iraq, Iran, Vietnam, Korea, Cuba).
Some good but mostly bad, Hitler's influence shaped more than half of the 20th century.