When Lucy is forced into her 'Nyu' personality, she is reduced to a basic level of intelligence, with a diminished vocabulary and no access to her vectors. As the chapters progress, we see her begin to learn - specifically new words, like 'Kouta' and 'Yes'.
Early in the story, we learn that Diclonius children gain access to their vectors and begin to kill humans at around the age of 3. Given that Nyu is in a childlike state, and that she's learning much like a normal Diclonius child would, does that mean that she too would begin to murder humans after around 3 years?
Or would the fact that she isn't actually a child mean those consequences wouldn't occur?
I've already finished the story, so no need to worry about spoilers.
Side note: If I remember rightly, at some point in the later chapters (just before Kaede house is attacked), a particular event occurs where Nyu regains most of her intelligence, and can string together proper sentences. I can't remember exactly what happens, since it's been a while since I've read it, but for the question's sake, let's just assume that never happened, and Nyu stayed as she originally was.