In the endless argument about whether religion or atheism has caused the most bloodshed, believers wheel out Hitler (who was a Catholic), Stalin, Pol Pot etc while atheists bring out the Crusades, the 16th century wars of religion in France, Northern Ireland – to name just three. Stalin's the tricky one for atheists - although he trained as a priest, communism was an atheist movement. Mainly because he didn't want any rivals to his mind control.
However, the argument over whether religion or atheism has caused the most deaths is spurious. There will always be wars and dictators. Rulers and warmongers will use whatever tools suit them best to justify their actions. That's human nature. In fact, arguing over atheism/religion is just another in group/out group war.
Where do those leaders and conquerors fit who had a faith but did not use it to justify their actions? Alexander the Great slaughtered his way across half the known world without needing religious justification and the Mongols didn’t much care what faith you were as long as you submitted. The Romans conquered Europe in the name of Rome, not in the name of any particular god – and then used religion as a post hoc tool to unite and subdue - choosing the one they thought would best serve. Mithraism was a contender but didn’t quite fit the bill although if it had included women and been more peaceful, we’d all be Mithraists now and Christianity would be at best a footnote in history.
The way to prevent wars and dictators is not to get rid of religion or atheism as we will just find some other justification. Animals fight over territory, resources and mates. The only thing that will work is to change human nature. Good luck with that.