He was one of many leaders who committed terrible acts to civilians around the time of the World War. He was known to have mass murdered thousands of Russian civilians (his own people) and his soldiers committed horrible acts towards Germans including rape. Stalin was power hungry and did what ever he could to advance his agendas.
Joseph Stalin should be considered the worst mass murderer in history based on his Communist regime that killed tens of millions of his own citizens. He was a terrible man and was hated by many of his own people. He should not have killed so many of his own people.
Yes, Joseph Stalin should be considered the worst mass murderer in history based on his Communist regime, because he showed reckless indifference if not hostility to human life. He did not care about the suffering that he caused in his own country. Throughout history, he is hardly matched by anyone else who came close to the suffering that he caused.
I do not believe that Joseph Stalin can be consider the worst mass murder in history, there are others that may give him some competition. For instance, Hitler, no one really knows how many people he actually killed or for that matter made them commit suicide. Some think that Hitler was attempting to rival Stalin's kill numbers. Another runner in this would be Saddam Hussein.
Although it is indisputable that Joseph Stalin was a mass murderer responsible for millions of deaths, he is not the worst murderer in history. Mao Zedong was responsible for many more deaths in the Cultural Revolution. Some estimates of Mao's death count are upwards of 40 million people. China has a larger population than Russia.