...and has been around for a long time, long before the so called '4 rules of firearms safety'. The importance of this rule is that the all the others often go completely disregarded once booze and/or drugs are mixed with gunpowder. I would be willing to bet dollars to donuts that booze, drugs or both were involved in this shooting:
Simply amazing how friggin stupid booze and drugs can make people, then again, some people need no help at all in that regard.
All the best,