Two good posts by Alex Nowrasteh & David Bier on “Politically Motivated Violence is Rare in the United States,” showing the death tolls for Left and Right killers. Here is their “Politically Motivated Terrorist Killers: Data, Sources, and Methodology”
One summary chart:
I have questions about the definitions, but this is a strong start to a serious discussion.
Related: Thought experiment: How would the Right react if a Leftist-CK had been assassinated?