Stalin. Killed everybody.
Mongols. Killed everybody and destroyed eveything, did not care for assets.
Spain. The real indian Killers.
Nazis. Gencoide at the expense of the war effort.
Rome. Unmatched bloodlust, just because.
Japan. Rape of Nanking.
Stalin in my mind is the worst - he did it to his own people in the highest numbers and not entirely during a war or while preaparing for a war,and th world was pretty modern whenhe was in power as compared to the Mongols or Rome.
There is no evil like a government.