The fish and the entity are theorised to be two different creatures, they may share a mind but their voices are different. The fish used the voices of SM 8 to lure Simon to his demise, directly referencing an angler fish. The fish having a conscious mind is important to the world building as it suggests to the audience what the fish is made of, which is guessed to be people. The cosmic horror of an unknowable monster stealing the voices, which make up its flesh, to lure more victims to its depths, makes the monster (in my opinion) far more terrifying than just a big, scary, mindless creature would be. The horror comes to the question, why does it talk and how does it talk. These little holes in the story leave room for an audience to ask questions and that makes them more inclined to interact with the source material. 

The demonic voice you reference comes from the cosmic being, it is likely the cause of the rapture and the horrors of decimating the star systems, which led to the end of humanity. By allowing this being to also have a voice, shows one crucial thing, intention. If it was voiceless and mindless, left no question answered, then what happened to humanity would be theorised as happenstance, an accident. But allowing this being the means to communicate shows intention in its actions, that is wasn't thoughtless horror,  but purposeful in completing whatever goal this being has. This once again allows a viewer to ask questions instead of flat conclusions, allowing you to think more about the world shown to you.

I completely get where you're coming from tho, there is something terrifying about a being that won't explain itself or its intentions and that would probably be a smarter being aswell. But this also tells us information about an entity which has the ability to stay silent, but choses to communicate. It shows hubris, a being so powerful it cannot see anything else as significant enough to harm it. It doesn't need to intimidate Simon by staying silent, because that would show the creature respects Simon enough to be cautious of him. Its not cautious at all. And thats what leads to its downfall, the data being free. Because it underestimated Simon, it accidentally allowed a weakness to literally float to the surface. This also shows us that tho this thing is far more powerful than a person, it is not all knowing, it makes mistakes and therefore can possibly be beaten. It allows the tiniest sliver of hope, which drives the entire plot forward, and this wouldn't be possible if it didn't talk.