Is there anything you remember learning about at primary school, as fact, and thinking "that's actually quite horrible"?
Here are some of mine which I remember, which showed my childish priorities:
Wartime evacuation; the thought of being made to leave your home like that. (Loss of life didn't even cross my mind!)
Medieval weaponry - the thought of it killing people for real.
The fire of London.
Vaccinations being weak germs put into you.
Certain punishments for children by ancient civilisations such as the Aztecs. And we thought smacking is bad enough these days!