What is the problem? He felt his life was in danger just like Zim.
Well, anyone can say it. The question is if the facts bear out a likelihood such that we believe him and think it was reasonable for him to be scared.
If we believe him that he was, and believe that based on the facts he was reasonable, then it is a full defense.
If we believe him that he was, but don't believe that it was reasonable, then some states call it an "imperfect defense" and it is voluntary manslaughter.
If we don't believe him that he was, but do believe it was reasonable, then it is still murder.