Why did my cat die?

Hi Dr. Marie, I know you are probably overwhelmed but I pray you give me some piece of mind..last week my very vibrant, active 7 year old female cat became ill literally overnight..ironically, on the very day I put down my other 18 year old cat..she became lethergic, wouldn't eat and drank excessive amounts of water..I brought he to the vet the next morning and after a variety of blood tests and xray, the vet said my cat was very sick, with suspected pancreatites..her liver was affected and she had high sugars and had to be hospitalized for fliud therapy..the vet said she was diabetic and that we had to get her stable with insulin..after four days she showed great improvement and the vet let her come home with me needing to forcefeed her and give her insulin..after a couple of days home she seemed to be getting sick again..her breathing became strange..her eyes became glazed and I thought her sugas dropped so I immediatly gave her syrup but it didn't work..I rushed her to emergency but she died in my arms before I got there..autopsy revealed she died from toxic shock..they said her pancreas was bad for years..my problem is that I don't understand what happened and I don't know why she was only being treated for diabetes if she was sick enough to die..would a different treatment have helped? Or would it have prolonged the inevitable death? I'm so confused, distraught, and just need some clarity..please help ease my mind..