Truth be told, I’ve been through a LOT and Lanza had choices to make either way. He was already way into the culture of death and he was WAY too into living a life where he was an empty vessel waiting to be use by a demon.
I sincerely think that he was possessed (does not excuse what he did to those children by the way since there is choice in choosing to do the work of the devil) and evil was acting through him.
I’m sure that demons sometimes work through mental illness.