I would go with Hitler or Stalin. What Genghis Khan and Alexander the Great did was normal in that time period. They conquered other lands and subjugated their people. They just did it on a larger scale. Hitler and Stalin brutally murdered millions of their own people for various reasons. That isn't even comparable to what Genghis and Alexander did.
Alexander and Genghis Khan didn't set out to murder people. They wanted to expand their empires. This was common of most rulers in the past.