Because cats are not pack animals like wolves, they are not always protective and defensive for their people, but
they are fierce territorial animals, and if creatures get in their hunting grounds and mess with their belongings,
there's hell to pay. The little boy was part of the cat's turf; he'd probably been marked by the cat with its jaw
glands, and he was part of the cat's terrain. So when the dog showed up and dissed the kid, the blades of death
came out. Sometimes mama cats will group a kid with its kittens and "raise it" as a member of the litter.
Obviously, the kid makes a great big, slow kitten, but mama doesn't mind. There again, that kid is HERS.