When a political party or a religion or even an individual like Bill O'Reilly stirs up hatred among millions of followers, some small fraction of them will turn to violence - cause and effect. I would look at statistics for proof, even if it can't be proven for a particular individual. I don't see near as much hate coming from the left as from the right. But there may be some disagreement as to what is hate and what is satire.
I enjoy anti-Trump humor, and I have posted a few crude and satirical images on the Facebook page Anti Trump Memes. https://www.facebook.com/groups/1774619239445679/ A Trump supporter (perhaps a troll) called it hateful. So I am engaged in a discussion with him on what is legitimate satire, and what is hate speech. One makes you laugh, the other makes you angry. No doubt he was angry at what I thought was hilarious, but I doubt his anger would make him want to kill the target of the satire. I doubt anyone who is anti-Trump would react with anger instead of laughter.
Bret Stephens makes an argument that political violence is just random, "indigenous American" violence, not caused by hateful rhetoric. He does not cite any statistics to back his argument, however. I don't have the statistics either, but it is my impression that political figures are much more likely to be targets of violence than other people who might have equal public exposure.
https://www.nytimes.com/.../virginia-shooting-james...Of course there could be factors other than the rhetoric. Maybe just being in a position of political power is a factor. Maybe seeing a cartoon of Trump in a bathtub with rubber duckies is enough to set someone off. We need some good social science to resolve this question. Then we can tackle the question of where to draw the line between satire and incitement of violence.