Thamiel as Justice-Maximizer
I have this personal theory about Thamiel, from Unsong.
Note - I apologize if this has already been discussed in comments at some point. I read all comments on a chapter when I first read the chapter, but I don't have the time/ability to keep up with all the discussion as it develops. I am eternally grateful to the commenters who link back to developed fan-theories in newer comments, they're super helpful!
Anyway. Thamiel was once a necessary and positive force within creation. An aspect of the divine. Thamiel was Justice.
"You convince God to save Sodom. And what do you get? A city full of Sodomites. The scum of the earth. Worms and maggots infesting the world. And now they’ll never stop, because you showed them they’ll never face punishment for their crimes."
Justice is a vital force in the real world. A world without Justice would be far more terrible than what we have. Thamiel's purpose was to make sure that Justice was served. That evil was punished, and deterrents preserved. But Thamiel was not created with proper limitations. Justice is the only thing he cares for, and he wants endlessly increasing amounts of it.
What's an effective way to increase the amount of Justice he gets to mete out? Cause more people to sin. The more that you can incentivize people to do wrong, the more souls you get to inflict "Justice" upon after death. So, in a perverse attempt to maximize the total volume of Justice handed out, Thamiel does his best to create the conditions that result in the greatest amount of people damning themselves. A horrific world of endless violence and despair. All in the service of what we would agree is a good principle. We want a Just World, but not Maximum Justice Inflicted. Much like we want enough paperclips to fulfill our needs, but not Maximum Paperclip Mass.
I'm not saying that Thamiel is an admonition to humanity to work harder on the AI Value Alignment problem, because not EVERYTHING has to be about AI. It could totally be a coincidence. After all, Some Things Are Coincidences Some Of The Time.
I'm kinda hoping that the second, withered head of Thamiel is later revealed to be the counter-balance that was supposed to keep this desire for Unbounded Justice in check, which was flawed in some way, and now vainly tries to protest with the minuscule scraps of energy it can muster. But that's pure speculation/hopefulness on my part. Can't wait for more Unsong!!