You are using words the meaning of which you don't understand, statutorily or under the common law.  Larson, properly under either, attempted to do his duty as a citizen by merely following Tallman to assist in his arrest and was assaulted multiple times without striking back. Only after the third incident did he present a weapon in an attempt to -deter- another assault. Tallman then deliberately chose to attempt to assault an armed, older man rather than using his youth and greater physical ability to flee and was justifiably shot. There's nothing "vigilante" in that fact set.