The answer for me is quite clear. As teens, all they knew about Hitler was what they might have read, but even more what they had seen in movies, TV, pictures, etc. Once schools succumb to this virus, it takes miracles to avoid some kind of violence by youngsters. Top Toy guns can get some kids expelled from school, even if bringing it were an accident. Not all boys ever played with toy guns.
Conventional masculinity is defined partly by a desire to be strong and competent.
But it is manifested in radically different ways. Some of those ways are destructive, some constructive. Those boys in Littleton didn't think of themselves as the bad guys, they thought of themselves as the hero-victims. One of the reasons I fled from the public school system was its obsession with victim-oppressor cults. The problem with victim morality is that victimhood is no virtue.