The murder of these two girls is an aberration in every way. I've googled many times to find similar cases with an elderly murderer. As you say, it's very rare.
Anyway, I did happen upon this "fiction novel". It sounds really good and I've added it to my wishlist. $10 is a bit rich for my reading pleasure.
https://www.amazon.com/dp/B0055UTQ4G/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1
Turn of Mind by Alice LaPlante
"A stunning first novel, both literary and thriller, about a retired orthopedic surgeon with dementia, Turn of Mind has already received worldwide attention. With unmatched patience and a pulsating intensity, Alice LaPlante brings us deep into a brilliant womans deteriorating mind, where the impossibility of recognizing reality can be both a blessing and a curse."