So sorry to hear the sad news.
We returned from vacation and noticed our kitty wasn't his usual self. Over the course of a few days he seemed to get worse, he wasn't eating or drinking.
We took him to the vet and they determined his kidneys had failed. We faced the same heartbreaking decision OP. At first I blamed myself for being away, but he was getting excellent care from a professional pet sitter. The vet said I shouldn't blame myself and it had nothing to do with our absense. He was almost 16. We were lucky to have him for that long.
Cherish the memories.