Well, it's uncommon. That part is worth highlighting.
Out of a population of 316 million people in the USA, school
shooters are extremely uncommon.
But... they happen.
Anger is common among young people ANYWAY. Between the ages of
18-27 or so, many young people join causes. Political causes are
common. Some kind of "something" to be angry about and some way
to solve it.
But... what if you have the anger, something to be angry about,
but no community to keep your rage focused in a productive
manner?