Perhaps it's a story about revenge. Maybe the mother believes that the child has done something unforgivable (even if it's only in her distorted perception) and she decides to take this extreme form of revenge. This would show how a normal relationship can be twisted into something truly terrifying in the realm of horror fiction.
It could be a fictional horror story. There are many such extreme and macabre tales in the horror genre that are designed to shock and horrify the audience. However, it's also possible that it might be based on some extremely rare and abnormal real - life events, but this would be very rare.
It could be on a mission to spread fear. Some aliens in horror stories are like that. They enjoy causing chaos and panic among humans. So they choose a street, a place where people feel safe usually, to shatter that sense of security and create pure horror.
One possible motive could be protection. The mother may want to shield her family from a painful truth.
Jealousy could be a motive. Maybe she suspected her husband of infidelity and in a fit of rage, committed murder.
One possible motive could be extreme revenge. If the husband has been unfaithful in a very hurtful way or has committed some heinous act against the wife, she might be driven to such an extreme act out of sheer rage and a desire to cause him the ultimate harm.
One possible motive could be some sort of twisted religious or cult - based belief. They might think that sacrificing humans in a slaughterhouse - like setting would please their deities or gain them supernatural powers. For example, in some fictional horror stories, cults sacrifice people believing it will open portals to other dimensions.
Some may create a 'homeless kid fake story' to gain attention. They want others to think they are kind - hearted or to make themselves look important in the eyes of others.
One possible motive could be to reignite the passion in their relationship. Over time, the spark can fade, and a wife might seduce her husband to bring back that excitement. For example, if they have been married for years and fallen into a routine.
Maybe he was a bit of a do - gooder in a very wrong way. He saw what he thought was a woman in distress and his sense of heroism kicked in. He didn't consider that there could be something supernatural or dangerous about the situation. He just wanted to 'save' her from the tree.
One possible motive could be madness. Maybe he had lost his sanity long ago and the sight of the woman in the tree was just another object of his deranged actions.