Legally justified use of lethal force in defense of another is not vigilantism. But to be justified, such use of force is subject to the same standards as for defense of self.
 But if one does use lethal force in what he believes to be defense of another, he better be sure he knows what's going on and that his use of force is legally justified. A mistake can easily send him to jail.
 On the other hand, vigilantism is never acceptable.
And let's please keep this thread on track. These discussions tend to wander, and if this one does, it'll be shut down.