Even Harry didn’t consider destruction. Even Harry.
The Fallen are to some degree capable of change when interacting with a human, I wonder what would have happened to Lasciel if she reunited with Lash, who had learned to love Harry?
In most cases the host is either dominated or in total agreement with the Fallen in either case this does not result in change. Namshiel/Marcone is going to be interesting Marcone won’t be dominated, nor are they in total agreement.
This ability to change may explain why one or more of the Fallen have thrown in their lot with the Black Council, plus their basic untrustworthiness, they partnered up with a human who couldn’t be dominated and with whom they were not in total accord, instead over time they changed their minds to follow the human agenda.
I theorise one of the reasons Nicky destroys records of the Denarians is that he doesn’t want people to know he doesn’t control all thirty, and he set up his wife’s faction as semi-loyal to hide this from the majority of the Denarians themselves, everyone in Nicky’s faction thinks x-iel is part of the other faction which is why they never see them, everyone in the other faction thinks x-iel is part of Nicky faction which is why they never see them. We have never seen anything like the full thirty assembled.
We may have seen x-iel or at least his host, the Fallen can keep hidden especially well, Harry may have fought him and not even realised he was facing a Denarian, Cowl for example is a likely candidate, he never pressed Cowl hard enough for him to draw on Hellfire in their battles. Cowl however knew Harry possessed Bob, who had the Word, if Justin was a previous host x-iel would know this. It always bothered me that Cowl knew about Bob when only Justin, Elaine and Harry knew this, so Cowl must have found out from Elaine, unless there was another being present all the time. Everyone goes with the Elaine-Kumori theory, but if she was bound to Summer at the time of Grave Peril, it’s unlikely given how we have subsequently seen how Winter binds.
Harry may be the first person in over a millennia to realise there is a third faction, it may only be one or two members and they are working with the Black Council and Anduriel cannot spy on them. Now there is a fourth.