I think I remember some years ago a zoo fed a dead giraffe to the lions and people went absolutely insane about it. I'm not sure about the details anymore, they might have killed it because there wasn't enough room in the zoo for it? Either way I didn't get it, what do they think the lions are fed any other day? Animals that weren't killed explicitly to become food? Some cows that couldn't bear existence on this world anymore and offered themselves as lion food? Where is the difference?
Considering how much meat is required to keep a lion fed per day, part of me thinks feeding it to lions would be sensible but on the other side it depends on making sure the meat is cleaned and that the animal that died didn’t die of a cause that would cause internal damage to the lion. Lions on the Serengeti feed on freshly caught food so their catch is usually pretty clean.
The zoo near me a few decades ago had an accident where a lot of their reindeer got pregnant before the male could get neutered. They kept all the female offspring but they couldn't risk having two males in the same enclosure and no nearby zoos needed male reindeer or also couldn't accommodate them so they had to put them down and ended up feeding them to the carnivores in the zoo.
Kind of related: The barn where my kid goes to for lessons has an option for the horse owners that in case their animal dies (there are strict rules about the cause of death etc), the animal's corpse can be donated to the local zoo to feed carnivores.