Yeah we'd all rather not have to cross the line of killing someone but in snap second life or death situations, the fact that someone dies three tenths of a second after hitting the ground from a doubletap to the high chest (solar plexus, or heart) or three days later in the hospital, is not a matter for concern. Abiding by the law, respecting the rules of shooting (be sure what is behind what/who you are shooting at, shoot to stop the danger, don't shoot to kill). Playing idiot is never wise.
Now granted not all mugging situations are life or death, but there are statistics that say that 'active cooperation' is more likely to result in death than active resistance, and plenty of muggers kill their victims after robbing them, so anyone looking to mug, in my opinion, has accepted they are prepared to commit suicide by crime and are okay with dying and so there's no reason to feel bad about it. So if they pull a knife and are pointed in my relative direction and they are not acting in self-defense and are not law enforcement, they are getting shot. No talking, no negotiating, no waiting. See knife, then point-shoot high in the chest (I can do on silhouette, snap from the pocket out to seven yards), they drop, then make the calls. And be sure to call the local police department(not 911) and let them call the ambulance. Don't be a criminal, don't have to worry