This is false. For example, the libertarian @cato.org breaks down the numbers, showing about 6 deaths from right-wing politically motivated attackers for every 1 from left-wing attackers.
Moreover, the claim that shooters represent anyone other than an unhinged fraction of society is the deeper lie.
Don Moynihan
Vance: "While our side of the aisle certainly has its crazies, it is a statistical fact that most of the lunatics in American politics today are proud members of the far-Left."