No war has ever been caused by religion.
Religion is often how it is sold to the people, but the cause is political or economic.
And now will come the masses crying "Crusades! Crusades!". The crusades began as an example of the primacy of domestic politics. And continued because you don't give up those trade routes without a fight.
Also, On Killing has been thoroughly trashed by psychologists and military historians. The basis of the book, SLA Marshall's studies, are questionable at best. He discounted all evidence in order to prove his ideas, and never let a fact get in the way of telling a good story. Grossman may be entertaining, but it's just a money book.