Humans are pretty terrible and we'll find any excuse to justify our terribleness. One of the parts of the French Revolution was the Dechristianization of France. While this may sound like a good thing, which should lead people to live their lives based on reason, it also led to violence against priests. And the lack of religion did nothing to stop the Reign of Terror. In short, it was less an atheist utopia and more just humans finding different excuses to be terrible to one an other.

Similarly, the Soviet Union was founded on the Marxist principal that "religion is the opiate of the masses". This meant that the Soviet Union was officially athiest. However, unlike some of the French Revolutionary governments, the USSR largely tolerated religious practices. At the same time, the officially a theist state got up to a lot of horrible stuff.

At the same time, there is an argument to be made that Christianity helped reign in some of the worst excesses of monarchs during the Middle Ages. It's important to remember that people really believed this stuff. Kings really did think about their immortal soul and what they would be forced to answer for on "judgement day". Fear is a powerful motivator and it may be that, for all their terrible selfishness, some monarchs may have been led to moderate the worst of it based on that fear.

All that said, I'm not sure how much differently history would have played out, without religion. As I led with, humans are pretty terrible. Many wars may have had a religious veneer, to get the people to go along with them, but they were more often about power, control and ego than religious conviction. Religion provides a convenient excuse to define "the other". The othering of people creates a permission structure where we will not only tolerate, but often gleefully engage in, truly horrible acts against "the other". And it doesn't require religion to do it. Take a look around the Lemmyverse and you'll find videos of Russian soldiers being blown apart by drone dropped munitions. And the comment sections will be talking about how "they deserve it" or making jokes and light of another human being ripped apart. And these comments will be defended because of the horrible actions of the Russian Government and some Russian soldiers. Russian soldiers have been placed firmly in "the other" and so we can celebrate their horrible deaths, and be cheered on for it in many corners of Lemmy. No religion required.

So ya. I'm not a fan of religion, nor am I religious myself. But, I have no illusions that religion has a lock on people being terrible to each other. It has absolutely been involved in making it happen throughout history. But, I am skeptical of the idea that history without it wouldn't have been just as filled with humans doing terrible things to each other. Human nature tends towards tribalism and the creation of "in groups" and "out groups". With those in the former more than willing to do anything and everything to the latter.