Who says or can prove dying on impact is painless? What if the reason you die is because it is the most painful experience ever.
Do you mean that the pain is so bad that the human body can't take it? Like, too much pain can kill you? If someone was shot I'm pretty sure they'd die because of the damage to their body parts, not because of the pain. But thinking that you can die from too much pain is a cool thought.
thier is a type of tree (I can't remember what its called off hand but I'll look it up) in the Amazon rainforest (and probably other places too?) that is covered in little sharp hairs all along the bark, if you touch them they sting you similarly to the way a nettle does, and their has been several cases reported of people dying from this. The pain is so intense that your body goes into immense shock and you suffer a massive heart attack and your lungs squeeze themselves shut, so it's definately possible to die from too much pain.
Yeah, but is it the actual pain that causes death, or the reaction from the chemical which causes the pain in the first place that causes the death?
Well yes it is the reaction to the chemical that kills you, but the reaction to the chemical is that it lights up your pain receptors, it doesn't have any direct effect on your heart or respitory system, its the pain caused by the reaction that kills ya, well technically its the exertion and heart attack caused by the shock, but the shock is caused by the pain which is caused by the tree. If your a little bit masochistic and want a clearer example of how this would happen, burn yourself with a match and observe the massive jump in heart rate and the inability to breathe normally for a few seconds, then imagine that only many many times more accute and pronounced and continuous.
Take strength from pain, Show you're alive In the case of a car crash you might even be strong enough to break out if only you could remember to use it at the time.