Individuals that are pathetic losers who wish to get revenge upon those that are not losers are the primary "cause". Locations, like schools, offer plenty of acceptable, densely packed targets and generally have lax security. I almost checked gun laws but that is simply part of lax security if those gun laws supply "security" in the form of a gun free zone sign. Imposing penalties for simply having guns on campus is never going to stop a suicidal nut determined to make a big splash in the press as they exit this world.
Black Jellybeans.
You might think that people who kill everyone are trying to look cool for media but I dont think most of them are. They are just sick of being picked on and bullied and not getting help when they ask for it. I tried asking for help in school before and I was ignored but by law had to keep going. Staff is suppose to do something about bullying and other issues but a lot of times they dont and hassle kids over crap. For example suspending a student for having a poptart shaped like a gun is not stopping gun violence, school shootings or bullying among students. Its just a load of crap.
Hilary Clinton's face.
Everything.