I just had the unpleasant experience of having a pet euthanized about two years ago. Actually for the same reason this thread originally started. A breeder sold Buster (springer spaniel) after beating him severely or knowing that he was born with a neurological problem.
Buster was dumped by his previous owners, my family and I made an attempt to take him in as a family pet. Buster was a fearful animal, fearful of men, his water/food bowl, leafs, everything. Towards the end, he had bitten my brother twice, unprovoked. We decided, that after a year of hard work that it would be best to have him put down.
He was taken to our vet where he was first given a sedative. The sedative made him start to have convulsions (proof of brain trauma, or genetic malfunction). Then, the "lethal" injection was given.
In the end we made the right decision. Clearly, this animal (through abuse or otherwise) was finally put out of his misery. I actually feel a lot of guilt because I tried for so long to help him. He lived a very unpleasant life, even after provided with a good home.