The source of the children's murderous hatred for their parents can be found into the themes of the story.
Among them, there is alienation vs. the needs of children. It is clear that the children in this story have been alienated by the veldt. The parents basically let them come and go as they wish and, most importantly, there is an overall lack of dynamics between the parents and the children. The consumerism under which the main characters are struggling to become better parents has taken over the wants of the children since their needs are not being met.
The alienation of the children results in physical and psychological abandonment. If we go by the Psycho-genetic stages as proposed by Erickson, certain milestones must be completed before reaching another social and psychological stage. Notice how this goes perfectly with what the children are experiencing: they are literally being left to their own devices while the desperate need of being cared for, of feeling loved, and of belonging somewhere continues to be neglected.
Hence, the children substitute their needs with wants; the wants now become the whims that come to their imagination. Since they already feel distant from their parents, they transferred their longing for love and converted it to pure wants and whims...Why not get rid of them altogether? Their need for them has ended. They do not need a home either, why, if they have the veldt to feel safe?
Therefore, what once was love, safety, care, and company has turned into abandonment, alienation and the satisfaction of needs with wants. The rest becomes ingrained resentment and, ultimately, rage and hatred.