The same was true for reasons, other than wars, for kids throughout history. War, famines,and disease carried off many a parent throughout all of history, yet there weren't mass shootings by teen boys.
This latest shooter was adopted by far too old childless parents. His mother was 49 or so when he was adopted, his adopted father died 10 years later, and supposedly he was never "right" to begin with.
There’s a difference between not having a father because he died, and not having a father because the mother just wasn’t into him anymore. The difference to the child is quite profound, psychologically: the first is something that happened due to acts of God, the second is something that was done to the child on purpose by human beings, often for the lamest of reasons.