The hunting instinct is independent of the urge to eat. Cats with bells on their collars do kill birds. Birds do not necessarily associate the sound of a bell with danger, and cats with bells learn to stalk their prey silently. Birds that seem to escape don't get away unharmed. Wildlife rehabilitation centers report that a large percentage of their patients are cat attack victims and animals orphaned by cats.
Most animals injured by cats die.
Cats carry more than sixty types of bacteria and viruses in their mouths, some of which can overwhelm a small animal quickly unless medication is administered immediately. A victim that looks perfectly healthy may die from internal hemorrhaging or injury to vital organs. These losses are totally unnecessary because, unlike other predators, pet cats don't need these animals for nourishment. Drastic change, short of supernatural intervention, is rarely long lasting. Lifelong habits are not easily broken.