As a general rule either a cop or "civilian" may only use enough force to stop the attack anything else is excessive use of force in the eyes of the law. Example shortly after midnight you hear class breaking and go to the source of the niose to find to thugs one armed with a baseball bat the other with a gun razing his weapon to firing position. Being the prepared citizen you are, using your trusty 1911, you double tap the guy with the gun in the chest who falls to the floor. Having to think fast as you don't know the intensions of the other fellow, who is holding a deadly weapon, you apply the Mozambique Technique (two to the chest one to the head) you then notice the first scumbag trying to raise his gun and pop him in the head (Yo Homeboy is that my briefcase?) Depending on your locality you will probably be charged with murder and depending on the jury convicted. Afterall you shot one guy when he was on the floor, mortally wounded, and you killed the other guy outright when all he wanted to do was play a friendly game of baseball! Then if the perps had any family at all they WILL sue you reagdless of any criminal charges! In the above example I would consider the actions of an armed citizen to be perfectly justified and reasonable since all of this would take place very quickly in many localities so would a grand jury, In other places the citizen would be cursified. If the citizen were white and the perps black Jessie and Al would crawl out from the baseboards to cry racisim. Bottom line is if possible the best course of action is to jus just enough force to stop the attack BUT knowing just how much that is is no easy task and you may likley face legal reprocussions even if your actions are perfect.