It's because we no longer have a functioning democracy.
It's the gerrymandering. It's the twin Supreme Court rulings from hell, Citizens United and the roll back of Voting Rights. Politicians play to their base and their big donors while ignoring the majority. Our votes don't count. Politicians don't listen. Rich get much richer and everyone else gets screwed. Regular people are getting really angry. This is very predictable.
Democracy is a compromise we make to avoid bloodshed. I do not condone violence against elected representatives, but more is inevitable if they don't do something to fix problem soon. Revolutions and 2nd Amendment remedies may sound exciting when they are just part of some political rhetoric, but in reality, they are scary.
The Supreme Court needs to step up and fix their mess by doing something about partisan gerrymandering, to start. We, the American people, need to re-dedicate ourselves to the compromise we call democracy. Make all the votes count again and then abide by the results of elections peacefully, even when we think it sucks. If we fail to do this, prepare for some really nasty shit.
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they disagree with. Shooters have been white supremacists, Islamic extremists or disgruntled for some personal reason. That I recall. There are so many random shootings these days, hard to keep up 
Trump is an incredibly polarizing figure. Combine that with the hyper-partisan congress, a divided electorate and too many guns, it is not a big stretch to think that things could get very ugly very fast.
Computers and big data have turned gerrymandering into a different beast. The tsunami of dark money is a corrupting force. The roll back of Voting Rights Act left the door open for the voter suppression like we haven't seen since before, well, Voting Rights. We have a President and controlling party who are incredibly hostile toward free speech too. Free speech is the mechanism, other than democracy, that we use to avoid violence. It's the safety valve, but with the lack of representation and leadership from out 'elected' leaders, it is not going to be enough.
People are very angry, even the kind of people who don't generally get really angry about politics. We could write this guy off as just another gun nut who ran off the rails, but I worry that he is the tip of the iceberg. Hope I am wrong.
Ironically, I read an article the other day where a RWinger was quoted as saying that politicians didn't respect liberals because liberals don't threaten to rebel (I believe he meant they don't threaten to shoot people with guns for political reasons). Wonder if he feels differently now? Maybe I'll go back and try to find the article....
