The existence of large Christian, Jewish and Yazidis within Arabia since the dawn of Islam. If they were behaving the way they are right now for a millennium, this wouldn't have been the case.

The Sharia law in Daash is basically identical to the Sharia law in Saudi Arabia which is based on Salafi Sunni Islam, but more specifically, on strict dependence on the Sunnah of al Wahab. Who himself lived a couple hundred years ago, but whose perspective didn't gain much international prominence till the Saudis pushed it around the globe.

My background is Catholicism and Judaism, though I grew up in largely Muslim parts of Russia (Urals) and western India without any problems at all. To be fair, the latter were mild, and the former, lackadaisical. But I am, very happily, an agnostic theist.

Though even Muslims who take Islam seriously are somehow not automatically the sort of people are causing problems. There are many interpretations of Islam one could take completely seriously and never because a terrorist. Just as many third-wave Jihadis aren't particularly, religiously knowledgeable.

Obviously, there are many religious Muslims who hold very dangerous views very closely to their hearts. The vast majority of these people, as far as Sunnis go, adhere to that Saudi Salafism mentioned above.

Islam isn't going away. In fact, at the rate it's going, it'll be the world's largest religion a few generations. This not something to necessarily be overjoyed about, but it the reality.

So, since it's not going away, the best hope for a peaceful world would push out its most violent and in-plural interpretations. And those have become prominent in the 20th century.

So, hopefully, turning the clock back, along with promoting Sufism, is the way to go. Because we're not going to be able to shoot our way out of this.