For the time period and what he was able to do I think Attila the Hun was the worst tyrant. Could you imagine what he would do if he had the mechanics of World War II?
Genghis Khan - he's alleged to have killed 40 million people, directly or indirectly. Entire cities and kingdoms were wiped out. Mao had 20th century technology and got about the same score. Imagine what Genghis with machinegun wielding Huns could have done.
What about Alexander the 'Great'. He may sound great for western cultures but for everyone in Asia that was subjected to his men he wasn't that way.
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.
Edited: Kyrroeth on 15th Oct, 2017 - 1:29pm
If you look at Hitler or Stalin as the worse I would have to go with Stalin as the worse. He did it for no other reason than the fact that he could do it.