Folks act out based on models they see in part.
Look at the guy who was flying the Cessna so that the USAF would shoot him down. If it weren't for 9/11 - it wouldn't come to mind most likely.
Social cognitive learning theory clearly shows us that people can observe the outcomes of actions and mimic them.
The repeated coverage of Columbine, VT and other rampages reinforces the next rampager. He or she is driven by their underlying pathology but models their action on what they learned.
Every memorial, weeping parents and friends and pundits discussing the rampager reinforces some next one who sees him or herself generating the pain, being psychoanalyzed by some schmuck Dr. Phil, etc.
Legit experts have been beating the drum of not doing this media blitz but it falls on deaf ideas.
When a school has a big gathering and everyone waves candles, Mom cries on TV how Victim Biff and Tiffany were good kids (he was on the team and she was a cheerleader), the police chief thunders that the shooter was a coward and Dr. Phil-oid says he was disturbed (and if only we helped the poor soul) - the pathological person is vicariously rewarded for planning the next action. Seeing Mom cry over Biff and Tiff is rewarding to them.
Unfortunately, the world of sensationalist TV - it won't stop. How many other shooters and Octo-Moms (ban hi-cap Moms?) are now planning their actions?
That's what's going on - not the looney stuff (antigun plots, Supreme court, immigrants, etc.)
1. Underlying pathology
2. Life stresses that exaggerate the stresses and increase patholology (so losing your job, going nuts over politics, problems as an immigrant, being ditched by YOUR LADY, etc. are examples of a stressor)
3. Anger at some group and/or society - felt picked on or not supported by something or someone.
4. Modeling a violent action - so that they become an expressive killer and/or plan a suicide with hostile intent. That can come from video games, watching coverage of past rampages, focusing on weapons related media. They can focus on gun culture and weapons. More exposure to weapons probably interacts with these folks to enhance their violent ideation.
5. They intend to die a 'warriors' death' as compared to a lonely suicide - they want to make a point with their death - Revenge, change society to take into account their concerns (a weird and perverted altruistic motive).
That's the package.