What needs to be remembered, as incredible as it sounds, is that the vast majority of the perpetrators of this crime were not crazy, or animals, or suchlike. They were people like you and me, who, given a particular set of circumstances lost all trace of humanity. As such, they serve as an eternal warning against extremism, and the apathy of those who stand by.
I run an Israeli-German youth exchange programme, something unthinkable 60 years ago, but with the will of good people on all sides, there is no limit to what can be achieved. The first lesson is for all of us to grasp (and of course many in this world don't) exactly what Jack tells us; that first and foremost we are all human beings.