I cant believe a vet will just kill your animal cause you say so....it is absurd to me and abuse. I have had this talk in other threads this is my opinion. I think it is wrong if the animal is perfectly healthy anyways.
Most reputable vets won't put down an animal just because the owner says so.. they'd at least talk to them first and possibly suggest other options. I can understand that perhaps the owner wants the animal put down because perhaps it is hyper-aggressive and refuses to respond to training (rare.. but I have seen a few instances of this and from a liability stand point, its safer to put the animal down than risk it escaping and hurting someone.)
But the point here was that neither the vet nor his/her technicians questioned why this "healthy animal" was supposeably being euthanized. Even just one of them asking the owner "Are you sure you want this cat put to sleep?" would have prevented the entire tragedy.