Whether they're influential or not is strictly subjective. He was associated with them.
As for Atkinnson, he was movitaved by a desire to kill liberals, Democrats and homosexuals. He could have been affliated with any number of right-wingnut organizations, from local militia groups to Aryan nations. He cited right-wing radio shows as pumping him up.
The point is that there are groups and pundits who enable the psychos and then sit back and feign sorrow, then denial that the pyschos listened to them.
It's a tad bit significant that we're not seeing a comparable level of bad behavior from the left-wing side of the aisle despite all the insistence that there is.