This video contains a lot of good information for the armed citizen. John from Active Self Protection hit a lot of good points in his assessment of the confrontation. The only thing I have to add is the likely reason the armed citizen was not charged with a crime in this shooting despite his actions that resulted in the death of one assailant.
One man can not effectively defend himself against the number of attackers in the video. Based on the video evidence, I feel that it is safe to say that the armed citizen was facing deadly force. Had he not been armed, he would have been able to protect himself. Disparity of force reasoning can be applied to multiple attackers who place one in fear for his or life or it could be a sheer strength and size equation. A 220 lbs man could surely inflict deadly force upon a 110 lbs woman (in most cases). Those are just some examples of disparity of force.
Obviously these principles do not apply in every country or location, so do your due diligence in understanding of the laws of your land and how they may effect your ability to defend yourself.