Why did my red ear slider turtle die?

well i had a red ear slider turtle named yoshi. he had shell rot a few months ago so we took him to the repti vet and they gave us treatments and we treated him as instructed and we took him back and he was healthy. so a week ago we took him just for a check-up and they said he was fine and today i came home from my friend's house..and he was dead..why? his housing was perfect and he had everything he needed and he ate regularly and pooped just fine (ew) why did he die?