Your argument has a fatal flaw. If a leader is a theist, he could be any of the thousands of religious sects out there. If he's nonreligious, he is one of two things: Nonreligious, or atheistic. Some people consider both the same. It is simply stupid to try and compare solely Christian deathcounts with atheist deathcounts, because seriously, there have been a lot more atheists than Christians. Go tally up ALL deaths motivated by religion, and compare them to all deaths motivated by atheism - you'll theistic murders to be a lot higher.

Also, most religious leaders kill in the name of their god(s). Most atheistic leaders do not.

Napoleon was a deist, and still described religions as "man made," but he did believe in a distant and uncaring god, and revered Catholicism, though it is mostly unclear whether this was out of required respect, or because of the immense social greatness he believed it brought.

Kin Jong Il believed he was a god; he was as brainwashed as his followers were. I would not call him an atheist.

Stalin was a psycho. Most if not all of his atrocities were committed because he was extremely paranoid. Little of it was due to his atheism, and, as I already said, the man was insane. Blaming an entire group of people for what one insane follower did is stupid.

Now on to Hitler. Truth be told, that paragraph is leading me to believe you're a troll. Hitler believed in a very distorted version of Christianity; that is, he believed in an Aryan Jesus that fought against the Jewish. Mein Kampf gives several insights into Hitler's religious views, mostly that he believed he was doing god's work. But Hitler also had mental issues, and his view of reality was heavily distorted by drug abuse, so I personally would not count his actions representative of Christianity as a whole.