There is, imho, quite a mixture in here.
A fish that is owned as a pet does not compare with fish raised for feeders, at least in the owner's eye, or it should not.
It is not the eel, or whatever. It is a fish that has been cherished as a pet and then, just because one wants to change track, the easiest way is to kill it.
How many of us are stuck with true monsters that are unplaceable? That nobody can provide for?
Killing them would solve the problem, would free tanks, would open a lot of new possibilities.
I, for one, cannot.
But i understand some may think otherwise. But 99% of those are poeple who have fish today, will have canaries tomorrow, and a hamster the third day.