The world is the inevitable result of a god that requires sacrifice
Humans are ingenious and industrial. We will always seek, and find, ways to make processes more efficient, and require less direct labor.
Now apply that to a world where a literal god exists and demands living sacrifice. Not only that, but the more senseless the death, and the more suffering that is involved, the better.
This is why there is no apparent purpose to anything seen in the underworld, just lots of apparently meaningless drudgery, prolonged suffering, and senseless death. Humans created the underworld as a sort of perpetual sacrifice machine, it's only purpose is to create sapient beings with only as much awareness as is needed to work internal processes and understand their own demise. Everything physical is recycled back into the machine because it is the suffering that is offered to the god.
Perhaps at some point the creation of this machine itself led to the apparent death of humanity as a whole, or perhaps once humans created the machine, the god saw no reason to keep humans around and killed them—all except for The Last Man.
The Last Man might believe he is trying to destroy the machine, perhaps under the impression that without sacrifice, the god will die. However, his role adds material to the machine in the form of The Assassins, and with that material, more suffering. The god might well understand that, eventually the machine would run dry without external input, and so has allowed one human to live, and believe whatever he wants, to continuously add metaphorical fuel to the offering fire.
The Alchemist may have began as a part of the endless cycle of sacrifice but eventually reached a level of awareness that drove them to seek an end to it as well, and now uses the endless Assassins to create a divinatory reagent that they hope will lead them to that goal.
The first Assassin we follow was able to triumph in the end
The one that was in the medical ward? That's the one who was in the car and motorcycle. The Assassin's bomb actually did go off in the end, which is what the explosions in
The Stinger were
- But the assassin in the car arrived at a different spot than the one who was operated on. It's implied in several scenes that the old man on the surface is continuously sending assassins down.
If
God Is Flawed, then so is his creation. Eventually the flaws became too great, humanity wiped itself out and what's left is a
World Gone Mad full of senseless suffering. So God creates the Assassins with the intent on literally blowing the world away so that he could start fresh. The Alchemist and his deformed friend discovered this and uses The Assassins — having been created by their Mad God whole-cloth — as a means of starting fresh by initiating their own Big Bang.
The diving bell appears to go down, but is (counterintuitively) actually going to the hereafter. It's not necessarily "Hell".
The Last Man is literally the last
man. He sends Assassins to the afterlife to destroy it and free the souls it contains. (The bomb briefcases are so small relative to their effect because there needs only be so much amount of Earthly material to do the job.) The skeletons there are embodiments of the dead; they are literally kept as trophies and decoration. The shit-men are temporary vessels for the souls of the dead, doomed to toil in the afterlife for some inscrutable purpose, and the master of the factory might not even exist except as a means to make their labor even more pointless. The Assassins contain within them jewelry and gold and documents as a means of warding off external (unearthly) influence by fortifying their existing mortal material with the products of humanity. Unbeknownst to the Last Man, however, the Alchemist has actually been using a living component of the Assassins to "restart" their realm ...but because it's still being run by the same powers, it keeps ending up as a hellhole.
- The warscape is some form of Valhalla, but for the embodiments of wars rather than any mortal souls.
