There does not need to be a crime for "justifiable homicide" - for instance, it can be the killing of another in self-defense when danger of death or serious bodily injury exists, even if the other person does not know they are posing said risk - I will grant you such force used must be reasonable and cannot be excessive. If there is some reasonable step short of deadly force, that should always be used first. Let me know if you have any further questions.
How is a woman's life in danger if she has a baby that is the result of rape or incest? How would that be justifiable by your definition?