Even with training and domestication, pets have been known to turn on their owners. Sadly, this is all the more common with exotic animals that are difficult or impossible to train, such as lions, bears, and snakes. These stories are tragic, usually of devoted guardians attempting to simply do what can't be done - tame a wild beast.
Pets who have turned on their humans include a famous chimpanzee, various reptiles, a hippo that inspired a TV special, bears, and even spiders. Some of these cases are the result of obvious bad ideas, but many were just animal owners who had fallen in love with an exotic creature and paid the ultimate price.