The Sunnis who perpetrated this atrocity have no humanity. To have humanity means you are not selective about which children you kill. If you believe that we are all God's creation then how can a man bring himself to do this? Perhaps he does have a natural revulsion to decimate life in this way but goes ahead anyway, or perhaps the killer has developed a taste for it.
Killers like this are impervious to any reason or any law, because they consider themselves to be Right Men on the Right Path. To be in a large group of men who all think they are special, chosen and the best must be very intoxicating, and only those who can dismiss the evidence of their own eyes when they know that many good people go about their lives being kind and cooperative to all; they see this in their neighbourhoods and towns, but that's still not good enough because You Are Not The Same As Me.
I think it is easy to get brainwashed in this sort of atmosphere, and to travel quite a way into jihadism. There is something seductive about feeling a martyr; we talk about XX being a martyr to housework, a martyr to working overtime, a martyr to her kids, etc. The inculcation of young men & women begins with firing up resentment, and just like the unscrupulous can help to establish false memory in a person, and cause anger in the subject, the same with clever hate preachers. Resentment against non-existent - centuries old and recent - events or situations are stoked up over months until the subject is so full of resentment and hate. Those who have been brainwashed no doubt experience a warm glow from having a shared feeling they have been put upon, no longer will be and are now going to right the perceived wrongs done to them. You have a jihadi.