Stalin killed many people both through his direct rule e.g., the purges, and also indirect such as the artificial famine in Ukraine.Anglo-American historian Robert Conquest estimated around 20 million people died but he even said that it was too low. Other despot's such as Hitler may be more infamous and his genocide may be ruthless but in terms of statistics, Stalin killed the most. Stalin also left a regime built on fear,lies and murder.
That's a pretty ignominious title, and it's hard to say who was the worst, but Stalin was certainly of them. Stalin is responsible for the torture and deaths of 40 million to 60 million people, and for imprisoning many in gulags where they were treated so incredibly badly. He is clearly deserving of a spot on the 'worst' list.
I know that the atrocities the Russian people faced during WW2 and even know is largely covered up and because of that Stalin may be considered the worst despot. Stalin was terrible but I don't think anyone was as bad as Hitler. I can't imagine any human being trying to eliminate a group of fellow humans so methodically as Hitler did. No one is as worse as Hitler.
It's hard to pick the worst despot of the 20th century, because there were so many to choose from. Hussein is another person who made countless people suffer terribly. It is hard to decide what makes one despot worse than the other, but Hitler was the worst because he had the worst intentions of all of them.