Girard does not embrace a Hobbesian view of the world, he is trying to explain the origin of human cooperation and society. The scapegoat narrative is not a nihilistic war of all-against-all it is a narrative of all-against-one to create order. Human cooperation can exist when violence is pared-down to one person (the scapegoat).
Where Girard admits that nihilism can come into play is when the scapegoat narrative becomes openly acknowledged. Which is what he argues Christianity effectively did (opened up the scapegoat mechanism for all to see, thus rendering it ineffective). Girard predicts that our lack of a scapegoat mechanism will likely lead to apocalyptic violence.
Frankly, I think the leftist view that humans can cooperate out-of-the-head-of-Zeus is naive and needs no account, because it has never happened.