The only way I am using the defense of other clause is if I am defending those I am with, or in a situation where I am in the store when the robbery commences to occur. I am not a sworn officer of the law, I have no "duty" to protect others. My duty is to myself and my family primarily. My duty to others is to call the police and be a good witness.
It's people with the Captain America, I have to save the day mentality that gets them in trouble.