I'm surprised he only got a passing mention in a single list, but Ivan The Terrible is way, way up there. He threw dogs, cats and other creatures off buildings as a kid for his own personal amusement. He would sentence random people to horrible deaths for no good reason other than his profound paranoia, watch the executions with glee and even partake in some executions. He'd basically murder anyone and anything, including his son, his unborn grandson, and his wife after one day of marriage.
To top it off, he was an absolutely dreadful leader (say what you will about Genghis Khan, Atilla, Vlad Tepes and even Hitler, but they were at least somewhat successful). Ivan triggered several costly wars that lost him territory and through his incompetence, indirectly lead to the Time of Troubles when his legitimately retarded son took the Tsar title after his death, which triggered a famine that wiped out a third of Russia's population.