Lionfish do not see someone coming up to aquarium and then hope they stick their hand in. They are not aggressive fish, however the sting (depending on the type of lionfish, and each individual persons toxin tolerations) can cause a sharp pain in the part of the body stung, usually the hand. In some cases the stings have been known to cause paralysis, swelling or death. However, there have been very few cases of death due to a lionfish sting.
As far as stinging fish, if you think about how the sting can effect you and then think about the size difference between you and a fish, you tell me what you think would happen. It all depends on the size and activity of the victim.
It's just like a rattlesnake bite, when I got bit if I had been exerting large amounts of energy and had been 30 lbs lighter, i could have died or gone further into whatever it is it did to me. So, moral, whether it can kill you or not, don't stick your hand in the tank. (or your pet percula)
P.S. lionfish do not sting to kill prey, they sting in self defense. Actually they probably don't even know that they have stung.