Who are you including? Socialist guerrillas in Latin America? Islamists?
I went searching and looked into where this idea of mine came from. My statement is a little out of date, stemming from 2017 so I'll retract it partially. If you look at this Washington Post article, and scroll down to the bar chart, for 2016 amd 2017 you'll see 11 killings by the right wing extremists and 16 killings by the left wing extremists.
https://www.washingtonpost.com/national/in-the-united-states...
However, you'll note that the number isn't zero. It's just that the mainstream media is very left biased, and engages in the kind of "not technically censorship" suppression of information typical of the West, as called out by Noam Chomsky in Manufacturing Consent. Note how effective that style of soft suppression was in preventing your awareness of those killings.
It did remind me of the congressional baseball game shooter, who as far as I remember, didn't kill anyone (though not for lack of trying). That was stacked against several more successful mass murder attempts from right wingers.
What we should be concerned about, is how the dishonesty and bias distorts the information available to the public, and destroys civic discourse.
I still think antifa is blown out of proportion for propaganda purposes