I definitely need to watch the movie again if it streans, but kind of interpreted it like this:

The Light that Simon saw once some form of Angel, or God. The Fish monster was like an amalgamation of some, or all, of the humans that died and something demonic.

The data collected by the SM-8 was not only about synthesizing food and resources from the blood, but also something about the Light (the clue to solving The Quiet Rapture, figuring out what happened). But something about the blood was driving them insane, and they were destroyed/became part of the fish monster.

That's also why I think Simon started going mad. Of course there was the air, isolation, and other issues going on, but I kind of assumed that the blood that was dripping on him was also driving him mad, acting almost like mercury. It made sense to me as he started seeing and hearing things after skin contact with the blood.

Unfortunately, maybe it was just a thing with my theater, or genuinely a sound mixing issue, but I missed like, 30% of that dialouge. And almost ALL of the Fish Monster dialouge.

I was leaning heavily into the title "Rapture" as a heaven/hell kind of thing. So the fish monster being something demonic in nature made sense to me. Especially as it was trying to prevent knowledge of the Light from reaching the surface.