Why did my cat eat its only kitten?

This is very sad but can happen. Sometimes the mother thinks that the kitten does not belong to her, or she doesn't want it, in some cases this is even because they are so hungry they will eat anything. If the kittens die of weakness/illness the cat may eat it as it is nutritious, just as they will sometimes eat the placenta/afterbirth. If this continues to happen, you may want to consider getting the mother neutered.