There are some big veins and arteries running through there not too far from the surface. If your assailant gets one you are on a pretty quick trip to the morgue. As others have mentioned, sepsis is a real problem—if your intestines take a hit, the normally helpful bacteria in there can go straight into your bloodstream. That is a painful, nasty, and prolonged death.
Just needs to emit some bone-cracking noises as its joints go the wrong way to make a normal person vomit and faint