Why do lions kill cubs?

Male lions sometimes kill cubs. It works like this. A coalition or group of male lions take over a pride of lions. They kick out the other male lions, kill the cubs of the females and then procreate with the females to create their own offspring. They kill the cubs so the females become sexually receptive almost immediately. They need to do this quite quickly because another coalition of male lions will come in, in about two years or more, to kick them out. So they have to do it quickly to ensure that they protect their offspring. The objective of all this? The male lions want have their line of offspring, their family. Not all male lions achieve it. The bigger and the more virile they are the better the chance.

It does not always happen that males kill the cubs of the females in the pride that they are taken over. The female might be pregnant and she can then disguise the offspring as the offspring of one of the takeover males. Or she might be able to hide an existing cub and then raise a new cub and the existing one together as if they were the offspring of the incoming male lion.

Alternative description

Not all lions kill cubs. Male lions sometimes kill cubs. Male lions want to leave offspring of their own. They do this when they have tenure of a pride of lions through a coalition of male lions. The average length of tenure of a pride in the Serengeti is only 26 months. Some male lions, the smaller ones for example, don't get the chance to mate. Killing small cubs after a takeover of a pride reduces the time a lioness takes to produce new young.

Image by Ian Lindsay from Pixabay 

A lioness becomes sexually receptive almost immediately after losing her young and can conceive as soon as two weeks after the death of her previous litter although the interval is usually much longer.

So, when a coalition of males take over a pride the incoming males kill small cubs and evict older young, thereby inducing females to come into oestrus and mate. This makes sense from the males' perspective. It is infanticide. It represents the best chance for a male to leave offspring of his own.

Males killing cubs is a way of increasing their chance of having their own offspring. 

Also because cubs are vulnerable to infanticide for 2 years an incoming male sires his cubs as fast as possible so he can protect them by increasing the chance of being around i.e. before the next takeover of the pride.