first you have to make an argument. :)
Hundreds of millions of people have been killed by atheist commies. Stalin, Mao, Pol pot, Kim il Jung.
Someone who believes that man is nothing more than an undesigned collection of chemicals probably won’t cry too much if such a ‘collection’ is arbitrarily ‘rearranged’ (i.e. killed) by another collection of undesigned chemicals.
That’s exactly it.
Two worldviews.
One sees humans as the “crowning glory of the Creation of God, made in His image, with the purpose of glorifying God, and having eternal existance”.
The other, as you say, sees no purpose in human life, existance for the short number of years they are alive, and of no more value than any other random collection of chemicals.
Now, which worldview is going to care more about the number of humans killed?