Call it a hunch. I have seen "this kid" hundreds of times on the streets and in the parking lots of shopping centers and malls just as you probably have too. It's unfortunate that this particular kid crossed paths with somebody so deranged as to actually act on his frustration and anger in such a permanent way.
I do recall once when a couple of friends and I were nearly hit by a guy running a stop sign. About a mile down the road, the man pulls us over and flashes a badge and tells us off about speeding, which was false.
None of us said anything but after he left one of my buddies said the guy was a distant cousin of his and was really mean and had a bad temper. The boys Father had a talk with the sheriff and was assured the reserve deputy would lose his badge.