I am a psychologist and generally good at perceiving reasons driving behaviours. I am not a criminologist and have not been involved in the Letby case, but I can see a possible "motive" having read about the case and watched Panorama. It is just a theory.
I don't think "killing the babies" was the ultimate purpose of the behaviour.
Lucy had a very close relationship with her parents, had a difficult delivery and was obviously fed the idea that the heroic nurses and doctors saved her life. She wanted to be a nurse from childhood.
I think she wanted to be a hero. She wanted to be remembered by parents as the exceptionally heroic nurse who raised the alarm about their baby's condition, went out of her way to sit with them, comfort them, write them cards, make memory boxes. She reflected how tragic the situations were with colleagues in her texts, again seeking affirmation of how selfless, marvellous and heroic she was, carrying on regardless despite being in such sad situations. The way she seems to have especially targeted multiple births, IVF cases reinforce this. She can get affirmation about how terribly sad the situations were. I think she really didn't see the babies as people, just as disposable pawns in her quest for heroism and adulation. It's incredibly twisted thinking.
So I guess a kind of FII, but not about medical attention directly, but about wanting to be thought marvellous, selfless, coping beyond the call of duty...just like the nurses she has always been told saved her life at birth. Maybe?