A central aspect of Giard's thought is how mimesis creates rivalry and how that rivalry gets defused through scapegoating.
The basic dynamic behind mimetic rivalry is simplicity itself. The central observation is that humans imitate each other (mimesis is the Greek word for imitation). But this tendency among humans inexorably leads us into rivalry and competition.
Girard describes this as a process of triangulation. We see somebody desiring an object. Due to imitation I begin to desire the same object. And with both of us desiring the same object a rivalry forms between us. This--mimetic rivalry--is the root cause of violence.
I agree with Girard about all this. But I'd like to argue that mimetic rivalry is not the fundamental, root dynamic. Specifically, Girard's model of triangulation presupposes a backdrop of scarcity. There is only one object of desire but there are two people. Thus, there competition between the two people to obtain the single object.
But what if there were two objects, one for each person? In that case, in a situation where there is "enough" or surplus, we don't have triangulation. In abundance we'd have mimesis but no rivalry.
My point here is that the anxiety produced by scarcity is what makes mimesis violent. That is, mimesis does not necessarily lead to violence. But mimesis does lead to violence in a world of real, potential or perceived scarcity. Thus I'd argue that anxiety, rather than mimesis, is the root cause of violence.
I'm not enough of a Girard scholar to know if this is an observation he's already made and addressed. My guess is that he has.
Regardless, my argument here is that fear is the root cause of human sinfulness.