This is a complex issue: whether one has a duty to preserve life in the face of a lethal aggressor or is there something good about refraining from enforcing one’s desire to live. 

I see aggression as something to be resisted by all those capable. I feel defending self and others against aggression is a duty, both a moral duty and  a civic duty. Our bill of rights agrees it’s a civic duty almost explicitly and implies it’s a moral one as well.