LAST EDITED ON 09-19-12 AT 02:32 PM (EST)
As much as I tried to find some definitive numbers for this, including a breakdown by religion (or atheist), different sites posted different numbers based on different assumptions.
I ended up settling for this op-ed Who Kills More, Religion or Atheism?:
And that truth, as I've said time and time again, is that people are people. No matter what segment of society you look at, you'll find good people and you'll find bad people. You'll even find, as has been said, that the line between good and evil cuts through every human heart. Certainly there are people in the news who kill in the name of religion, but just because they kill in the name of religion doesn't really mean they kill because of religion...
Religion is a convenient banner for many to carry, but there are plenty of other banners available as well, and if it wasn't religion, they'd do their deeds under some other justification, if they care to even have one. The real reason they do their evil deeds is that they're human. Humans are very smart, very capable; and when we want something, we generally find some way to get it, even if that means killing someone or committing genocide.