There's nothing nice or heroic about putting a dog down, quiet the opposite in fact, it's a very distasteful business.
That said, I would rather put a bullet in good blood than see it go to a wanker.
Pups can have the best breeding in the world behind them but it don't mean they will turn right
Physical deformities, mentally wrong and as has been said, not enough of the right homes are just a few valid reasons for putting down pups.
I don't suppose the " any dog can be good if it's raised right" mob will agree, but there you go