A long time ago, I rescued a stray cat that was living -- no, existing; it wasn't much of a life -- in a small fenced-off space at the end of the alley in Boston where I then had an apartment. It took several days of feeding, inching closer each day as the little thing grew in her trust of me, till I could snatch her up, stuff her into a carrier, and bring her to my vet.
She was filthy. She was half-starved. Her long fur was so matted she had to be shaved over most of her body. Worse yet, she'd broken one hind leg above the hock some time ago and it had healed up bent outward, so that she walked on the inside edge of the paw. My vet almost cried when he saw that; he said if he'd gotten her right away he could have fixed it, but by now it was too late.
Well! That wee Piglet (so named for her enthusiastic appetite) could get around just fine on that crippled leg, could run like blazes on it in fact, and she settled in with me and my other cats quite happily. Her body filled out; her fur grew back in to a rich soft brown tabby and white; and she proved to be an affectionate, gentle cat. We were all so happy.........
Until, after several months, things started to go downhill. Piglet began losing weight, despite continuing to eat voraciously. Her energy diminished. She began spending long stretches of time lying by the water dish, alternately drinking and just resting her head on its edge.
A visit to the vet confirmed my fears: Piglet was ill. Specifically, she was diabetic. We discussed whether to try her on insulin; but my vet's considered opinion was that euthanasia was the kindest option. And so I said a sad goodbye to my little rescue. It grieved me, yes; but it comforted me to know that I had plucked her from misery and given her a happy life for its last few months