Religion

In actual history:

* The south-american civilizations slaughtered each other for prisoners that would then be sacrificed to appease the gods.

* A celtic grave has been found of somebody who had been executed. They had drilled holes through the flesh of his forearms while he was still alive and "sewed" his forearms together with thin hazelnut-branches.

* The animosity between Jews and Christians began with the problem that almost all of the christian holy scriptures were in fact jewish holy scriptures. This lead to a need of the Christians to otherize the Jews, to save the legitimacy of Christianity as a valid distinct religion. This lead to two millenia of economic and religious discrimination, and finally to racist discrimination and the Holocaust.

* The Inquisition, which was born during the Reconquista-period in Spain. They tortured and executed people who had the wrong faith.

* During the Renaissance, esoterics such as Giordano Bruno and Tommaso Campanella were imprisoned and sometimes burned at the stake. Which is a shame because once you cleanse their ideas of the supernatural superstition they are actually very progressive.

* The Thirty-Year War. 1618-1648.
At that time there was a Cold War in Europe, of Evangelicals vs Catholics. The Evangelicals were France, Britain and half of the provinces of Germany. The Catholics were Austria-Hungary, Italy and the other half of the provinces of Germany. (Though catholic, Spain stayed neutral this time.)
There was peace. They lived separately but equal.
The people of Bohemia were Evangelicals with a tolerant catholic ruler. But when he died of old age, the Habsburgers insisted that by right of some international contracts they had the right to pick Bohemia's next king. And they picked an extremist who began cracking down on Evangelicals in Bohemia right-away.
The people of Bohemia protested and demanded the right to elect a king. They picked the duke of the Rhineland-Palatinate, a young, open-minded and idealistic Evangelical who had just married the daughter of the King of Britain. The duke was talked into accepting the crown, with religious arguments.
And with both power-blocs claiming the same crown, a bloody war broke out that lasted 30 years.

There is a line from Berthold Brecht's "Galileo Galilei", which is set during that period:
"In Germany, they slaughter each other with Bible-verses on their lips."

* Then, after WWII their was this tiny problem with the founding of Israel and the resettlement of the european Jews to Israel and how muslim Palestinians already lived there and how these two have been at war ever since.

* 9/11

* And then there was ISIS, which co-opted a secular revolution against Syria's dictator, thereby delegitimizing any opposition to the dictator.
Oh and ISIS also executes people who don't obey their cherrypicked version of Islam. And they enslave the Yazidis and have almost completey wiped out this ethinicity.
And they have destroyed precious archaeological artifacts.