Some depend on your point of view. Grandpops brought up Saladin, Islamic guy in the Crusades. It was however Richard the Lionheart who on more than one occassion whilst they were negotiating the release of hostages on either side just massacred towns instead (at the siege of Acra he killed 2700 hostages whilst both side were talking terms). There's a few massacres to his name whilst he bankrupted England (he famously said he would have sold London if he could have found a buyer) - not in the league of some of those mentioned but I'm not sure he was any less bloodthirsty or mental than Hitler, he just lived at a time when murdering millions wasn't a realistic possibility (he did however have the Jews who turned up to his coronation to pay their respects whipped and beaten to death). For my money Pol Pot. Though the Sierra Leone guy was a ruddy terror too!
Stalin and his ilk killed their own people whereas the British went around the entire world killing swathes of people for greed, destroying superior cultures and imposing a religion that few of them actually abided by. Hard to say who's worst imo.
Mussolini was born in Rochdale and had a broad Lancashire accent .... all his speeches were dubbed. Not many people know that but I have a photo of him playing dominoes with George Formby .... you can clearly see Il Duce knocking. The fella was apparently partial to a drop of stout. please log in to view this image
Theresa May not ever make a decision in her political career that hasn't been controlled or written down..