There has always been that social outcast that others ruthlessly picked on, but today social media amplifies it. Add to that an obsession with violent imagery that was not available several decades ago, stirs fantasies of revenge for that small segment of youth that can't distinguish reality from fantasy. Many movies and games are quite disturbing with extreme violence. It definitely has an effect on the mind of an already disturbed individual.
With America's large population, a fraction of a percent of the population becomes a lot of disturbed individuals.
Today's drugs, both legal and illegal are also more widespread and a large portion of the shooters have been on anxiety medications, most likely mixed with alcohol and illegal drugs.
There is almost always clear and present behavioral warning signs of most of these kids. Yet, despite obvious mental issues, many are afraid to intervene and our mental health system and legal system fail to protect the rest of the public from people who are a danger to themselves or others.
Media plays a big part as a suicidal youth will now seek notoriety by outdoing the last, when years ago they might have just killed themselves in anonymity.
Our education system also fails young males, who by nature are restless with short attention spans in a one size fits all approach at teaching in schools that aesthetically appear the same as a county workhouse. They are disengaged and bored.
JMHO.