Satan has no real being; he exists always as a parasite on the being of humankind, just as theology tells us that he exists as a parasite on the being of God. Satan is imagined and symbolized as a person, as "someone," because satanic power becomes attached to the victim as the victim mechanism does its work. The victim is viewed as a devil or demon.
Girard is approaching this from his anthropological viewpoint, and he tries very hard to stay there. I think it's a both/and situation when it comes to Satan; we are responsible for our own evil, and can self-initiate like anyone else. Satan is not the originator of evil, but the first to practice it, and encourages our desire for it. But I'm fascinated by the implications of this, and if Satan needs 'our permission' to enter into a realm expressly given to our stewardship.
