I don't think that insight is original to Girard. Nietzsche reinterpretted morality along those lines, i.e. the slaves don't have anyone to vent their will to power on, so they vent it on themselves in the form of morality. Freud's development of the ego and id are also along these lines.
True, but see my response to grusome where I explain how I see their ideas working together. My understanding of Girard's work is based on the wikipedia article:
You are right, I haven't read Nietszche state the precise idea that religion is based on scapegoating. Nietzsche does have the more general idea that religion is based on catharsis reached through violence and oppression. However, the root of violence is not mimetic for Nietzsche, but rather a common will to power that all have, but only some can act out externally.
At the same time, I can see the will to power idea working together with Girard's mimesis idea. The slaves want to be like the masters. This is the first form of mediation, the type that is beyond the reach of the desiree. When multiple slaves want to be like the masters, then you have the second form of mediation.
At that point, Nietzsche and Girard branch apart. Nietszche thinks the slaves reach catharsis through internalizing their will to power in the form of conscience and the ascetic ideal, while Girard thinks they reach catharsis through externalizing their desire for power on a common victim.
Yes, Girard thinks it is rivalry that leads to scapegoating, but if the original desire is power, then this neatly conflates the nature of the desire and the rivalry, since rivalry and the final scapegoating can also be considered the exercise of a will to power.
