For nearly 20 years I've said that organised religion is just another way of asserting power over people. Quoting from you:
> [...] two sides simply cannot coexist in peace.
That is divide and conquer played out to near perfection. The two sides are supposedly the same, but due to their own religious dogmas, they hold an existential grudge against one another. Those who wish to maintain and exploit the situation can pick any number of small clashes and turn them into straightorward propaganda.
And while the strategy is from the Roman emperors, the tactics are (as stated in the article) definitely from Stasi: surveillance, suspicion, paranoia, disappearing people and assassinated leader candidates. Even potential ones.
I wouldn't be surprised if the IS forces allowed a small number of individuals to escape from each of their massacres. What better propaganda than the rumours spread by few lucky survivors?
As to their units' training and composure... Again: isolation and suspicion, coupled with unwavering loyalty. There's a dirty word for that, and it starts with B.
Which, incidentally, would complete the circuit and bring us back to religion.