Why did my mother mouse eat one of her babies?? :(?

Mother animals often eat the sick or dead babies. This is not heartless, it actually protects the healthy babies from getting sick (baby rodents often die of wet-tail, a very contagious and deadly disease) and keeps the nest clean of rotting corpses. It also provides the mom with extra nutrition for the babies. The only other reason could be the mother is incapable of caring for the babies. This instinct to eat the dead young is why mice, hamsters, and other such animals have survived.