Why infanticide happened in animal world?

Taking good care of cubs is animal parents’ responsibility. However, the behavior like infanticide do exists in the real world. Why does some animal kill or eat baby?

The behavior of infanticide can happen when the leader of a pack changed. The new pack leader would kill the entire old one’s offspring. Then they will mate the females and produce its’ own cubs. It often happened in lion’s pack. Here is a video record the entire process. Sometimes it will happen in macaque pack, too. This is kind of a nature rule that eat the baby not their own. But why some animals kill their own children besides above situation.

In some extents, parents will consider their eggs or cubs as easy food. This can be happened at when the environment is extreme bad that they cannot get food for months. Male polar bear will kill baby polar bear; even it is their own blood. There always a fight between male and female poly bear at this time. Sometimes female will eat the sick one in order to keep others alive. However, sometimes the animal infanticide behavior is so weird that it seems like they kill baby for no reason.

Sources: http://www.livescience.com/2053-animals-eat-offspring.html

http://news.nationalgeographic.com/news/2014/03/140328-sloth-bear-zoo-infanticide-chimps-bonobos-animals/