This is the strangest thing for me. Not her appearance or that she's young and female.
I watched an old Allitt documentary on youtube, which I suppose is the case that bears most similarity, I was a teen in 91 and didn't remember a lot about that case.
Even with Allitt there were signs - she self harmed for sympathy, was known to get violent even beating up a boyfriend at one point. She had a longstanding reputation with friends and family for telling lies. Even after she was first questioned and removed from the hospital her compulsion was such that she poisoned the dog and the son of a friend who was letting her stay with them.
I once worked with a woman years ago who was charming, sociable, fun but seriously 'off' and that became apparent quite quickly. She was sacked for some dodgy stuff (nothing as bad as murder) and eventually word got back this was a repeated pattern wherever she went. She could maintain a facade for a short while but it didn't last.
I am baffled that there were no signs with Letby and I don't think being an only child, chintzy bedroom decor or doting parents really count as real signs.
Things I would expect to hear about are other smaller scale deceptions and lies. Other more minor manipulations and cruelty towards others going back to her teens as a minimum.
In the period between her suspension and arrest (2 years I think) did she just stop all compulsion to cause harm? Allit didn't hence her going after the people she was staying with. If what Letby did is compulsive how did she just stop? Can she switch it on and off? What did she direct the dark behaviour towards for those 2 years?
It is a rather terrifying notion because perhaps there are individuals out there that are even better at the appearance of 'normality' than she was and will never be caught.