I mentioned this earlier, but I'll elaborate a bit more. My own concern is not whether the Kirk shooter was on the right or the left, it's that there is an historical nexus between people expressing counter-culture, anti-social attitudes and behaviors and leftist politics. The following is a quote from Irving How's (editor) Beyond the New Left, 1970.
Dohn (Bernadine, member of the Weathermen and former interorginsation secretary of the Students for a Democratic Society) characterizes violent, militant response in the streets as “armed struggle” against imperialism... “We're about being a fighting force alongside the blacks, but a lot of us are still honkies and we’re still scared of fighting. We have to get into armed struggle.”
“Honkies are going to be afraid of us,” Dohrn insisted. She went on to tell the war council about Charlie Manson, accused leader of the gang which allegedly murdered the movie star and several others on their Beverly Hills estate. Manson has been portrayed in the media as a Satanic, magnetic personality who held near-hypnotic sway over several women whom he lent out to friends as favors and brought along for the murder scene. The press also mentioned Manson’s supposed fear of blacks— he reportedly moved into rural California to escape the violence of a race war.
“Dig it, first they killed those pigs, then they ate dinner in the same room with them, then they even shoved a fork into a victim’s stomach! Wild!” said Bernardine Dohrn.
Dohrn wasn't the product of online culture, and she wasn't an isolated male. She was a highly intelligent participant in her own youthful radicalization. Likewise, Manson was neither left nor right, much less a selfless champion for racial equality. In hind sight, knowing now the facts of the Manson case, which was rapidly developing at the time, Dohrn's comments seem so unhinged as to be made-up defamation.
I think it's important to note this is not an inevitable product of leftism (that's a contentious statement that some disagree with but I firmly believe) But it is a long standing historical danger. Others have mentioned it on this thread and I don't think it should be dismissed. The New Left totally collapsed in the early 70s. In fact Howe's writings were one of the reasons - exposing its essential nihilism. The left can and did reject New Left style radicalism. Dohrn herself went on to have a highly successful law career and later became a law professor. Her husband, Bill Ayers, likewise a New Left radical, was a professor and mentor to Obama who incidentally was allergic to anything that had a whiff of New Left rhetoric.
But to escape the danger, violent rhetoric and ideation and extravagant language must be forcefully and continually rejected. That's not happening (IMO) on the left, it's being ignored, minimized and on occasion even justified.
I'd add that it's wrong to identify this as solely and uniquely an American phenomena: it's international in the same way the New Left was (counter culture, youth oriented); and also that the right has a problem with violence too, arguably an even greater one, though the dynamic is different than on the left.