Any number of things, I have a viewpoint that most aren't going to agree with, its not a gun problem, nor is it a mental health problem.
It's a media problem compounded by a group of individuals who have a distorted sense of reality and perception.
The other factor is the term "mass shooting", these events are in reality "public suicides with mass killings". Most of the other events are familicide or felony killings.
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You can seek out that document if you are so inclined.
The demographics of the shooters are most often identify them as white males however the immigrant population accounts for a high percentage based on their population in the country (Pulse night club, UVA, San Bernardino were all immigrants)
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What is consistent is the fact that most of these people are reclusive loners with multiple life failures or feel they have been"wronged by society or a specific demographic".
They probably not been successful at anything in their life, spend most of the time on the internet talking with people who have similar views or playing video games (it's not the violent videos games either) they would have been otherwise isolated or outcast from society and are feed a narrative by the media. They are successful in their minds playing the games and the feedback from the game or other players is a huge boost to their psyche. They believe that these acts give them a sense of purpose and meaning and have a distorted sense of reality and perception and probably have sensory processing issues, hence the wearing of hearing protection.
I would argue that most of them are depressed, despondent and isolated, they're probably suicidal(but lack the willingness to actually commit suicide), the copycats and subsequent acts that follow one event are likely the motivation that they need to act or they want to "be more famous than the last guy".
When I say they don't have a mental health problem these people are not diagnosed is having bipolar disorder, schizophrenia or the other traditional mental health illnesses, probably more likely to have some form of high functioning Asperger's, ADD, OCD, or other diagnosis that makes it difficult for them to understand or interact with society, they probably have been seen by the mental health system, however this is not a person who is hearing voices and being told to shoot up the Walmart.
If you want to decrease the number of events, stop talking about it 24/7 online and on TV, don't mention the shooters name, and replace the narrative with "if you go to try and shoot up a public place, you'll most likely be killed before you succeed".
I'll stop before I write a essay.