christian (who had to live with a Muslim family

don't forget Christians and Muslims are literally cut from the same cloth - both are pretty much just offshoots of Judaism. Judaism itself is just a random tribal religion (of which there have been countless thousands) that somehow didn't get dropped and forgotten about as humans moved into the modern era. It's frankly weird that over half the world's population belong to one of these Jewish sects (and also a bit funny that there are only a handful of million who follow the OG Jewish religion).

And yes it's frustrating that so many people are so stupid as to base their lives around something that collapses under the slightest objective scrutiny, and probably not surprising that a not-insignificant number of them want to literally kill followers of all the other offshoots.

The problem is, there is inherent human need not to think. Some need it more, some need it less.

If tonight some magical device removes religion from all of history, we will simply find out whet other idea will those people latch unto. Would it be nationalism? Would it be cult of personality? Perhaps trusting only single media group, or discarding any research paper not done by "racially pure" group? Would they claim "how can we trust foreigners? if they do not follow our leader, they cannot be moral, right?"

Though I do wonder if things would be even worse if those people didn't have religion claiming there's external punishment or reward if they control themselves a bit.

Though when Christianity tried to resolve the "if I cross the line into eternal damnation then I might as well do whatever I want after that" problem by saying they can repent, it opened up a "do whatever you want and then repent" loophole, so it could do as much to make people think they can get away with shit, especially when combined with a strong belief that the afterlife will be better.

Also, I think the shitiness is ingrained deep in our species, hell in most species. The reality is that strategic betrayal has always been the optimal solution to the prisoner dilemma, and while real world versions of it continue after that betrayal, I'd bet a high portion of historic animal ones involved the one intending to be nice or trusting not surviving, so I'd guess that survival of the fittest has a bias towards opportunistic assholes.