While it's clearly bullshit to claim that Robsinson was a leftist, it also seems dishonest to paint him as right-wing. Especially his motive remains shrouded in mystery and certainly can't be boiled to "Kirk wasn't right-wing enough" at this point. It all seems super ambiguous, there's no simple explanation there.
https://www.kenklippenstein.com/p/exclusive-leaked-messages-from-charlie