No, a true atheist wouldn't have murdered millions of people
What does murdering millions of people have to do with being an atheist?
All that's required to be an atheist is a disbelief (or lack of belief if you want to argue that) Stalin definitely had that.
Stalin disposed of clergymen, condemned religion and turned the USSR into a godless state. While atheist would like to claim Hitler was a Christian who used religion to persecute Jews I'd like to affirm that Hitler only used Christ as the perennial victim of the Jews, comparing him to the German republic and people after the fallout of World War I, scapegoating the Jews under the false pretenses of Christianity. Hitler was also documented of having anti-clerical/religious views, adhering to Darwinist views, even contemplating on eradicating religion. Stalin eradicated religion because communism was founded by atheists who deemed religion as "opium of the people".
He believed himself to be a god more so than believing in nothing
That's stupid of course atheist would murder that's what the devil wants