I deal with gunshot wounds to the extremities often. There are complications from losing too much blood, transfusion reactions, blood clots, fatty embolisms, contamination of the site, tissue toxins affecting the liver and heart, septicemia, and with the amputation the same complications compounded. The stress from the trauma and resulting surgeries can be too much. It amazes me that the success rate of military trauma surgeons is as high as it is.
And with massive blood loss, you can throw in brain and lung function as well. Bodies don't function well living on Lactate Ringers.