why jack was able to act in a wild manner after asumming the mask

Jack was able to act in any fashion he wanted once he "assumed" the mask. What this means is that he could shed any facade of civilized behavior and thus revert to the savage behavior that is at his core. He no longer has to act the proper English schoolboy that he has been in the past; he can digress to the barbaric behavior that seems to come naturally to him and he does.