Bit of a theory, but I noticed that most ghosts will repeat the same mutterings over and over. What if this actually has some weight on the story, and it's actually what their last thoughts/words were before death? Or what they were most fixated on in life. But maybe the ghosts that were violent or committed some great sin when they were alive end up being more powerful or violent as ghosts; like the axe ghost. His phrase is "found you" and if my first theory is right, then he definitely did something pretty bad before he died. I really hope that we get some chapters that give us details on the deaths of some of the ghosts we've seen.
I think most souls only linger as ghosts if they were evil in life, or if they suffered a horrible death. A big example of the opposite could be like with Mieruko's dad; I think he stays by choice on the Earthly plane to be with his family and watch over them. But maybe these other ghosts are trapped, and need some kind of closure or fulfillment to pass on. But some ghosts have no hope, so they go to the thing in the wall that eats the ghosts as a kind of suicide. Maybe it's a like a worst-case thing, and if they are eaten by this wall thing or killed by those more powerful ghosts, their souls are destroyed forever.
Then again, maybe the thing on the wall doesn't destroy them and that's how they pass on. Like that old man ghost earlier that wanted to give his wife the code to their safe. Even though his wish was fulfilled, he still did the suicide thing. So maybe these ghosts are just too stubborn to pass on, and the suicide choice is how they leave the realm permanently. But most ghosts want to tie up loose ends or wreak havoc before doing so.