We have an odd detachment from ancient wisdom and understanding with regard to the bitterness and resentment that can reside in every heart. It's a good thing to occasionally revisit the story of Cain and Abel. We don't need to get inventive to surmise how this guy decided to kill a bunch of people.
So how do we determine who Cain is among us? This society presently isn't built for that. Somehow - like cyber security or even defending in sports - this has to be more of a team (all of us) effort.
Too many of us are depending on institutions alone. Plenty of people walked through that section of sidewalk and there are businesses that are near that section of sidewalk that Cain drove through and nobody thought of questioning authorities. I'm just using this as an example and not placing blame.