My turtle died today. I'd had Bob for a little over a month and did everything I could for him when I realized he had multiple illnesses (pneumonia, septic, skin infections, parasites) despite having lived in the care of a "good" local pet shop for a year. When he started bleeding out his rear end last night, I called the vet for the third week in a row, and tried to get him to drink water. In the end, good care, agressive antibiotics, conscientious feeding, sunlight, and love did nothing to keep him alive.
A note on dying turtles, mine got extremely agressive, and I suspect delusional, snapping at things that were not there, hissing, and being far more active than I'd ever seen. Watch out for that. Also bleeding and diarrhea, but that should be pretty obvious.