By "they" do you mean Iraqis?
"They" have never killed each other on this scale since their country was formed.
The destruction of their country by an illegal US invasion is why they are killing each other today.
It's a logical consequence of murdering, maiming, and displacing millions of innocent Iraqis.
What part of that don't you get?
really
google al-Anfal Campaign sometime
Where did Saddam get the precursors for his poison gas and the helicopters used to apply it?
Same answer: the greatest purveyor of violence on the planet.
"Writer Joost R. Hiltermann has said the United States government and U.S. State Department was particularly important in helping their then
ally the Saddam Hussein government in avoiding any serious censure for the campaign and in particular the attack on rebels and civilians in the city of Halabja. Hiltermann writes; '
The deliberate American prevarication on Halabja was the logical, although probably undesired, outcome of a pronounced six-year tilt toward Iraq, seen as a bulwark against the perceived threat posed by Iran's zealous brand of politicized Islam."
Al-Anfal Campaign - Wikipedia, the free encyclopedia