1. Duty to retreatThe rule ordinarily forbade the use of deadly force by one to whom an avenue for safe retreat was open. This doctrine was but an application of the requirement of strict necessity to excuse the taking of human life, and was designed to insure the existence of that necessity. Even the innocent victim of a vicious assault had to elect a safe retreat if available, rather than resort to defensive force which might kill or seriously injure.2. Exception of the retreat rule