Would it not be a much better idea to kill Columbus, and prevent the 'discovery' of the Americas, thereby saving the population of nearly two whole continents? Can't be much more efficient with your time travel than that I reckon.
edit: Just read the interesting link you posted to 20th centuries bloodiest tyrants. I wonder why Johnson and Nixon are not in the third list, bloody tyrants who killed over 400.000 innocent civilians in the Vietnam war using horrible chemical warfare.