In two instances I have observed reactions to witnessing the infliction of severe pain that surprised me.
The first occurred when, I believe, I was eleven years old. I was an ear witness, so to speak, to an event that disturbed me profoundly. Several friends and I were playing with fire, burning model cars or something with cigarette lighter fluid. The father of one of my friends, the one in whose yard we were playing, discovered us. Ignoring the rest of us, he spoke sharply to his son, snatched him up and dragged him into their house. The child was obviously TERRIFIED! His wails were unlike any sound I had ever heard come from a human. Once they were inside, the sounds of a horrible beating could clearly be heard from where we were. It sounded as if the child was being viciously beaten on bare skin with a heavy strap. Of course, I do not know exactly what was going on. I do know that the child was being severely beaten and the sounds of the beating and the indescribable screams of the child almost made me vomit and stayed with me for days. When I would think of it later, I would feel sick and fear and dirty. The memory did not arouse me, more the opposite, but there was somehow an erotic component to it. I am not sure even a shrink could help me understand this.
The second event to which I referred above, was much lighter because it was another's reaction to fictional infliction of severe pain. When first married, my wife and I went to see the movie "The Story of O" with another young couple. I found much of the movie very arousing but was turned off be the more brutal parts. Leaving the theater, I was surprised that the girls admitted being extremely aroused and sloppy wet. Later, discussing it with my young, relatively innocent wife, I was even more surprised to find that it was specifically the more brutal scenes that aroused her so much. Forty years later, that still surprises me.
I would be very interested is any experiences you have had witnessing the infliction of severe pain and your reaction to it. I would be open to discussing this subject privately with anyone interested.