This is truly sad, and I am not at all making light of the loss of a young life, not to mention the devastation caused in Ms. Kelly's mind by the guilt she must feel over such an incident. The families and those involved need our prayers.
That said, one wonders if alcohol or some other intoxicant played a role because it is unbelievably stupid. Never, ever point a firearm at anything you don't wish to kill or destroy. Kevlar vest or no, you are still pointing a deadly weapon at a fellow human being. Wardell foolishly encouraged his friend to violate the most basic of safety rules, and Kelly foolishly complied.
All guns are always loaded.
Mechanical safeties aren't safe, and they cannot be trusted with a life.
Keep your finger outside the trigger guard until you are ready to shoot.
Never cover anything with the muzzle of a weapon that you do not wish to see killed or destroyed. That includes stuff on the other side of a wall, door, floor, or ceiling. Or a bulletproof vest.
Be sure of your target before engaging. Check again. You're probably wrong. Check it again.
Realize that a bullet will not necessarily stop in your target even if you hit it. Know your backstop.