Can you kill a guy by kneeing them in the butt?

Answer 1

Yes, this is indeed possible, the most direct issue would be the possibility of breaking the individual's back and severing the spinal cord. This is more likely to result in paralysis, but could result in death. The chances would be greatly improved if they:

a) had something inserted in their rectum that might spear them and cause some serious internal damage,

b) be peering over the top of a cliff,

c) looking out of the open door of a plane without a parachute,

d) balancing on the wall of a tiger's enclosure in a zoo taunting them 10 minutes before feeding time.