According to the Negeri Sembilan Wildlife and National Parks Department, the macaque (this is the common wild monkey that we see in the forests and gardens) that attacked the newborn baby was an alpha male. What exactly is an alpha male?
Every now and then, there will be bands of male monkeys who try to drive the alpha male out and take over the group. Usually they do not succeed. The alpha male goes chasing after the band and drives the intruders away through fighting or making aggressive noises. But sometimes the alpha male is unable to protect his family. When this happens, he is driven away from the area and his unprotected females are taken over and raped by the band of male monkeys into submission. One among them emerges as the leader, drives the rest of the males away and becomes the new alpha male.
The methodical killing then begins. In order to assume full control over the group, the new alpha male starts killing all the male babies. Some older male babies who are more mature and mobile may manage to run away and join a band of male monkeys but most are killed. The fearful pregnant females induce self-abortions by banging their stomachs against the trees to rid themselves of any foetus they carry. Only when the male babies have been killed off or chased away will the new alpha male take complete control over his new family.
So what probably happened in Negeri Sembilan could be that a new alpha male monkey had detected a caged female monkey in the house, climbed in when no one was around, saw a newborn human baby and assumed that the baby belonged to the caged monkey, and then killed it. We know that this is a tragedy for the affected family, especially the parents of the baby, but unfortunately, this is all part of the monkey's natural behaviour. For the Wildlife Department staff to track down and kill off this male monkey will not do any good if the people are not educated on the danger of keeping wild animals in their homes. Today, the issue is with the monkeys. What else can it be tomorrow? Tigers? Iguanas? Pythons?