Just mere life is not victory, just mere death is not defeat.

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21 foot rule

21 foot rule exists for a reason.

To expand on this video, the male had a knife in his hand, the officer began to maneuver in order to find a safe line of fire.  The officer had his gun at his side when the male began to charge him.  If the officer had his gun holstered he may have been stabbed because of how close the male got before the finally fell.  

In this situation, well placed shots are the only way to stop an attacker.  Simply shooting into the body and hoping you hit a switch is not the right way to go about it.  Even if you only get one shot off it better be the one that stops him in his tracks.

Also, just an observation, it is best not to back pedal with a firearm in your hand with a threat presented.  Understandably the officer was in an intersection which may have caused it's own issues the fact remains that understanding your battlespace is very important.