The duress defense does not ever excuse homicide. Conduct of the other party is no defense. Police Officers may use deadly force to effectuate arrest if there is a reasonably or an apparently reasonable, threat of death or serious bodily harm. Police Officers must have a warrant to arrest for misdemeanors unless the officer sees it happen. Private citizen's authority to arrest is limited to misdemeanors only.
Private citizens can never use deadly force.
Deadly force can never be used. Can be used to prevent a misdemeanor. A defendant is allowed to use no more force than is reasonably necessary. Trap or spring guns are reasonable methods of prevention of crime. One can always meet deadly force with deadly force.