I understand your position as I have probably had similar discussions with atheists and their claim that religion has killed more people then anything else.
Being that I am a Christian, I can look at what the Bible says about war and killing and go from there.
First, before trying to get into the numbers, it’s important to look at the doctrine of killing in Christianity. There is no demand for Christians to kill unbelievers like there is in Islam. In fact Jesus talked about “shaking the dust from your feet” or leaving those who don’t want to accept the message of Jesus (Matthew 10:14). However, there are verses in the New Testament which support defending yourself (Luke 11:21 & 22:36) or defending others (John 15:13) or that governments can punish evil and protect itself (Romans 13:4).
One other common misconception is the English translation of Exodus 20:13 “Thou shall not kill.” The Hebrew word that gets translated “kill” is ratsach. Ratsach actually means to murder or assassinate. In other words ratsach is unauthorized killing. It doesn’t mean all forms of killing because God directed Israel to kill at various times. The word used for this authorized killing is harag.
Now, just because a “Christian” leader authorized the use of force in a war, that doesn’t necessarily make that war a “Christian” war. I’m not an expert in history by any stretch of the imagination, but I’m pretty sure that wars like the Civil War, the Spanish-American, and WWI were not declared to “punish people for not accepting Jesus” or because the Bible demanded it. I know that the abolitionists cause (freeing the slaves) in the Civil War, weren’t brought out as a reason until late in the war when Lincoln needed a new boost/angle.
So while some reasons why some “Christian” leaders may start or get involved in armed conflict or war may be based in Biblical tenets (protecting the innocent or punishing the wicked), it would still be false to put all deaths in that war at the feet of Christianity or religion.
I know this gets into some degree of hair splitting, so that’s why I just simplified my response.
I had one discussion with a pacifist who claimed he would never use violence to solve a problem. I asked him if his inaction to use violence resulted in an innocent child or person to die, wouldn’t that make him an assesory to the death of the innocent, specifically if he had the ability to physically stop the primary agressor. That situation left him in a quandry and he ceased discussion on absolute pacifism.