In the story "The Veldt," why does Ray Bradbury have virtual lions kill real people?

In "The Veldt," Bradbury may have used the virtual lions to kill the parents to emphasize the serious dangers of technology.
In much of Ray Bradbury's science fiction, there is an underlying distrust of technology. What he seems to find the most dangerous is the temptation for people to depend on technology to provide intangibles such as happiness, love, or emotional comfort. This reliance is characteristic of the aptly-named children, Wendy and Peter (characters from the fantasy novel Peter Pan by J.M. Barrie), who have divorced themselves from reality as they attend a plastic carnival rather than dine with their parents. At home, they sequester themselves in their playroom of virtual realities. These children have lost the "human touch" of life. Lydia Hadley bemoans that she is no longer needed as a mother.

"I feel like I don't belong here. The house is wife and mother now, and nursemaid....Can I give a bath and scrub the children as efficiently or quickly as the automatic scrub bath can? I cannot."

Since the physical parental roles have been eliminated, there are few opportunities for George and Lydia to interact with their children. As a consequence, the children are undisciplined and given to indulging their separate interests, as well as thoughts of anger at their parents and vengeance against them. Without this adult supervision, Wendy and Peter gravitate to the enticement of technology's delights. These delights come to dominate their lives and become a force that the passive parents cannot defeat. Within the context of Bradbury's narrative, then, it is line with the theme that the virtual "kings of the beasts," lions, consume the parents.