I do not know of any far-right groups of killers. I wonder if the historically-challenged author of this piece can name any?
There is the left. The left organizes itself into violent groups. Whatever they call themselves--socialists, communists, Nazis, Antifa, fascists--they are only different "flavors" of the same underlying leftist ideology. Any member of any of those leftist factions can call any of the other leftist factions "far-right" but that does not make it so. They are all competing for the same followers.
Choosing to support any of those far-left groups is something like choosing which violent street gang to support--Crips? Bloods? M13? They are all worthless thugs.