It is not the left that did this.

Trump and the Republicans have made it clear that they specifically do not represent anyone who didn't vote for them. At the highest levels, they explicitly, openly take pleasure in the doing things that distress those who don't support them. They refer to the institutions we revere, EPA, the fourth estate, safety net, etc, in extreme disparaging terms. 

Trump is not the only one who called for Second Amendment remedies. They all cheered the Malheur occupation and those who pointed guns at the Feds during the Bundy confrontation. They are building prisons as fast as they can, reinvigorating the war on drugs (aka, the war on black people) and abusing even the most innocent of immigrant young people.

The Congress treated Barack Obama brutally and were derisive when our side resisted Gorsuch. Trump led the charge to subvert the president that we elected with intentionally dishonest propaganda about his birth, religion and integrity.

This has been going on for years. Arguably it started with Reagan and welfare queens, certainly by '94 with Gingrich, worsening relentlessly since 9/11. It became clear during the Obama administration that, contrary to the promise of democracy, liberals will never be allowed to be represented or have influence in our governance. This is being done by the right. It leaves people powerless, desperate and angry.

Violence is never the answer but many are people are crazier, less mature or think that it is. This guy was unbalanced but he was not tipped over the edge by liberal ideology. He was provoked, brutally, repeatedly and intentionally.