In my late teens we adopted a cat who was 10 or 11. We thought she'd be fairly placid in her old age, but she turned out to be a prolific hunter who brought home mice, rats, voles and even baby rabbits. Once I woke up to strange noises and found she had brought in a live mouse and was torturing it at the foot of my bed.
Our previous cat had been separated from his mother when he was far too young, so had never learned to hunt - occasionally he would leap ineptly at pigeons, but that was it. Going from him to this full-on serial killer was quite the shock!