I'll just do the cats that I have had of my own, not childhood kitties. As i only adopt oldies, there have been a few unfortunately.
Charlie - 14, she started fitting two days after surgery for an abscess, vet said it was unrelated, I am still unsure. We had her on morphine for 24 hours, but just after I agreed to keep it going in case she could get better she went naturally. Still feel guilty about that, as it meant she died alone.
Tigger - estimated to be 5, she had suspected FIP. She had fluid in her abdomen, but it wasn't fully positive.
George - ?? - elderly - to date, the only foster I have lost (although this will change at some point this year), he had liver issues.
Snowy - 14ish, she had raised liver enzymes for nearly a year before it developed into cholangiohepatitis - while this can be treated, she refused all meds.
Blackie - 14 - mammary cancer as her owner had never spayed her - all her organs were fine for her age though.
Ginger - 15ish - he went on the street, i will never know what caused it, but he did have a heart murmur, and as he had lived rough for 3 years, it could have been old age.
Pebbles - 11ish - we dont know what caused her death, the vet suspects a tumour in her intestines, but it didn't show up on x-rays done a couple of months before - sadly, she didn't ask if I wanted a postmortem at the time. She had battled quite a few illnesses though.
Tiger - 14 - again, we dont know for certain, but she had a limp that was either due to cancer, or chronic inflammation, plus two arthritic hips. She refused all meds, so when she started acting out of character and picking at her food, I let her go.