What animals kill their own young when the young don't seem fit to survive?I covered this on another Quora answer, and will copy the relevant parts here. My answer is non-exhaustive. For more examples, see the original thread, where many posters have contributed (What animals eat their own babies?).
An excerpt from The National Geographic (LINK):
Indeed, mother bears, felines, canids, primates, and many species of rodents-from rats to prairie dogs-have all been seen killing and eating their young. Insects, fish, amphibians, reptiles, and birds also have been implicated in killing, and sometimes devouring, the young of their own kind.
When mammalian mothers give birth, they must begin nursing their infants-something they can do only if they're healthy and well nourished.
But if, for instance, a mother bear in the wild gives birth to unhealthy or deformed cubs, or is unable to find enough to eat, she will typically kill and consume them.
"They become a resource, one she can't afford to waste," Barthel says.
A mother bear-or lion or wild dog-does the same if she can't nurse her cubs or find food for them. And if one of her cubs dies, she'll most likely eat it immediately, as Khali [the sloth bear] did. This nourishes her and has the added benefit of removing the carcass. "That way there's nothing rotting in her den which might attract predators," Barthel says.