Who is they? The Yamnaya never made it out of the Ukraine.

We know people often kill people. That doesn't even need explanation.

My argument is that it is thousands of years and dozens of mixed generations.

Why do you decide to participate in a thousand-years-long campaign to kill the neighbor who is also your second cousin?

There is no reason they couldn't have turned the other way and attacked their other second cousin.

My best guess is religion but it could also have also been a different kind of fanatic ideology.