I believe William's original motive for killing the kids was because of his jealousy toward Henry.
In the books, there were indications that William was spiteful of Henry (like the diaries he wrote in, explained in TSE). Seeing that Henry could bring life into creations. William was a genius in robotics but something about Henry's work seemed more superior than William's. This must have made him kill Henry's daughter Charlotte, the one thing that Henry loved the most. And taking away Charlotte life, it sent Henry into a deep depression. Doing that action made William feel like he had more power over Henry. But William wasn't done with making Henry suffer. In 1985 he killed the 5 children, which then led to the closure of Henry's business. And later on William discovers remnant from the possessed robots and want to become immortal and so on. That's at least what I think is why he started killing the kids in the first place.
