Euthanasia quotes from the BCSPCA Cat management manual:
pg.50 Euthanasia

"Euthanasia is defined as "The taking of life to relieve suffering"
It is the responsibility of shelters to euthanise cats that have a poor chance at having a good quality of life. Although euthanasia ia a difficult event for humans, the welfare of the cat must allways be the first consideration.

Cats must be euthanised when they suffer from:
*Poor emotional health as assessed using the "psychological record"
*physical health probelms with poor prognosis as assessed by the consulting veterinarian.
*Temperamnet problems severe enough that no proper match with an adopter can be made.
*A behavioural problem for which rehabilitation is not available.

The decision to euthanise an animal is a serious decision both for the animal but as well for staff morale. Knowing that the decision was made based on that animals chances at a good quality of life reduces the guilt, emotional stress and anger sometimes experienced by staff

pg.49 Fostering for Feral Kittens

Although many rescue groups adopt out "feral cats" successfully, it is likely those cats had been previously socialized to humans(ie:, not truely feral)
Free living cats will progress through re-socialisation faster in a foster home. Efforets should be made to place these cats quickly with experienced fosters.

Based on current research(or lack thereof) only feral kittens less than 8 weeks or less can be socialised. Using a technique called flooding, they cannot be told apart from pets cats by the time they reach the age of one. However, ferals tend to bond well to one person and retain fear of strangers. Adult ferals may be tamed enough to have contact with a colony caretaker but cannot live in a home as pets. The confinement and constant proximity to humans would cause them to experience on going stress and become habitual hiders

pg.53 Making euthanasia decisions

Euthenasia decisions must allways be based "the best interests of the cat". Some cats may be euthenised even though the shelter is not full because their emotional, behavioural or physical health is too poor and not treatable. All possible alternatives must be sought before killing healthy animals that have a good chance at a high quality of life. The psychological record described in this manual is a tool that can guide the decision to euthanise.