same here. we had a clown loach slowly dying due to white spot, it's skin was actually peeling off, and we just couldn't kill it. i guess there's always the hope it pulls through, and tbh, although i love my fish, they are fish, and don't last as long as other pets, but i was still annoyed that it died.I dont think I ever could kill a fish or any other animal, even if it clearly was the best thing to do. Which is admittedly quite selfish.
we've had a couple of fish that have lived and died in this situation, or similar. we have 4 albino cats, 2 have only one eye each and 1 has no eyes at all, we've still no idea what other fish did that to them, but the blind one is actually amazing to watch, it's like it sniffs the food out!!
some fish can be so resilient, so let it be. my dad keeps telling me fish feel no pain, but i've no idea if that is true...