At best, McCain is a useful idiot (for handing the dossier from Fusion GPS to the FBI). Of course the FBI knew about the dossier beforehand. So, McCain's role was to be the token republican.
At worst, McCain was involved in the creation of the dossier. He certainly has shown motive to participate.
As the last active Keating Five senator, McCain is an established sneak and has been formally recognized as having poor judgment.