Here’s why benevolent secular humanism (aka atheism) leads to genocide: If you don’t believe in anything other than this physical realm, then the only thing worth doing is trying to improve the one world you’ve got: this one.
You can improve this world in one of two ways: 1. by simply making yourself more comfortable or 2.(because nobody really wants to be that babyish and selfish), try to improve the whole world and make it a better place.
However, you’re not going to be here very long, and because it is hard to make the world a better place you’re going to have to be quick about it. It would be nice to take the time to convince everyone to help you make the world a better place, but people are pretty lazy and stubborn and they will probably not agree with you and your particular plans to make the world a better place.
So to get the job done, you’re going to have to use a bit of force to make the world a better place. Break some eggs to make an omelette, you know.
This is where the second part of being an atheist comes in handy. Because you don’t believe in God or an afterlife or such silly fairy tale stuff like the human soul, then you don’t have to worry about what might happen to your soul after you die. This means you can do whatever you like to whoever you like because you will never have to pay the price. (an aside: funny how atheists blame religious believers for devising a comfortable pie in the sky belief for themselves! Geesh! what could be more comfortable than atheism? You can do what you please and never have to be called to account)
Therefore, if you have to round a few stubborn people up, lock them up and eliminate them you’ll never have to answer for it.
Furthermore, if you don’t have a soul, neither does anyone else, so you don’t have to worry how many you bump off. After all, they don’t have souls either, so what does it matter if you kill them? It’s no worse than pulling weeds.