The whole Stellar death has caused many many speculations and theories. My personal theory is that she died (mostly) from shock and cardiovascular failure. Before she died, she was at a near mental overload and was freaking out. This probably led to a heart attack (although she wasn't gagging for air as most heart -attack victims would have.) Shock can kill. An example would be car accidents. Some victims die from car accidents instantly from the mental shock (I'm not quite sure what the correct terminology is) and not always from the physical injuries. Another possibility was that she was pushed to her max limit during the battle, and needed the medication/healing needed. It would have been impossible to adminstrate the therapy to her because she was in the middle of the battle, and when Shinn reached her, her vital systems shutted down.