Looking into this further, even making a threat is a form of violence. The person making the threat scares other people, causes disruption, etc. For the moment, the person has control over others in that others have to react to the threat. The threat, even if a "joke" without an action plan, can be part of a timeline of escalation.
I found this in an FBI Academy doc (linked below):
"In general, people do not switch instantly from nonviolence to violence. Nonviolent people do not 'snap' or decide on the spur of the moment to meet a problem by using violence. Instead, the path toward violence is an evolutionary one, with signposts along the way. A threat is one observable behavior; others may be brooding about frustration or disappointment, fantasies of destruction or revenge, in conversations, writings, drawings, and other actions."
Keeping that description in mind, we see this shooter:
* Made a threat at his school, apparently
* Didn't make the rifle team at school (Did he want revenge? Did he brood for years about the rejection?)
* Researched JFK assassination (did he idolize Oswald and think his notoriety was achievable?)
* Conducted online searches of many top-level politicians (fantasy fodder?)
Here's the link to the FBI doc about school shooters, but note it is a PDF, not a website:
https://www.fbi.gov/file-repository/stats-services-publications-school-shooter-school-shooter