An actor is justified in using force against another to protect a third person when (a) a reasonable person in the actor's position would believe his intervention to be necessary for the protection of the third person, and (b) in the circumstances as that reasonable person would believe them to be, the third person would be justified in using such force to protect himself. The reasonableness of the belief may depend in part on the relationships among the persons involved (a matter to which we return below).The actor's justification is lost if he uses excessive force, e.g., aggressive or deadly force unwarranted for the protective purpose.