We must first note that this question implies that the “great man” theory of history is correct. It implies that killing any one individual would dramatically change the course of history. There is another school of historical thinking that argues that it is larger overall trends in the world, rather than the deeds of individuals, that cause history to unfold as it does. In this view, Hitler, for example, was not the cause of World War II and killing him would not have prevented the war. If this second school of thought is accurate, killing any one person will not really change the course of history significantly.
If we do assume that killing one person will change the world significantly, the most likely person to kill would be Joseph Stalin. Many people would point to Hitler since they would say killing him would stop the Holocaust. However, Stalin was responsible for more deaths than Hitler. The people who died because of Stalin were less visible than the people who were killed in the Holocaust but they were, for the most part, just as innocent. When looking at Stalin, we must also consider his legacy. Without Stalin, we can argue, the Soviet Union might have been less aggressive after WWII. This might have helped avoid the Korean War and the Vietnam War. It might have helped to avoid all the nasty little “proxy wars” fought between communists and American-backed forces in various Third World countries.
If we accept that killing one person can change history significantly, killing Stalin would arguably be the greatest help to humankind.