Honestly PETA's shelters being kill shelters is the only respectable thing about them. I get that animals ae cute and all the bleeding heart stuff but sometimes it's just better to put the animals to sleep. If they cannot be rehomed, if they will just spend rest of their lives being sent from shelter to shelter, home to home, given up, taken in again, given up again, getting constantly traumatized about being abandoned and becoming even less likely to be able to rehome, is that any good?
Shelters are full of mentally ill dogs that cannot live with humans because they'll rip the grandma's face off if she breathes wrong but because people insist on not just putting the mad mutts out of their misery they spend years living in shelters, geting adopted, killing the family's pet cat, sent back to shelter, adopted again, biting the neighbor's kid, get sent back to shelter again in this forever cycle because we gotta keep every damn insane critter alive even though there is already way too many pets and domestic animals. Obviously you shouldn't just kill off every abandoned or abused animal you come by, of course they deserve a chance but sometimes if the animal is not suitable for rescue it is for the best.