It is common sense but that’s a high bar to clear for some. Part of the standard rules of firearm safety:

Treat every firearm as if it were loaded.
Never point a firearm at anything you aren’t willing to shoot.
Keep your finger straight and off the trigger until you’re ready to fire.
Keep the weapon on safe until you intend to fire.

Some amplify rule 2 with “know your target and what lies beyond it” which is good advice too. Follow all the rules at all times and your risk of negligent injury to yourself or others is massively reduced.

Setting aside the possibility that this was an intentional and malicious act (which I wouldn’t rule out), the killer broke all four rules. Says he had two cups of wine too but that’s also the typical answer you hear from somebody on a DUI stop before they blow a .26 on the breathalyzer.