Chaosmancer
Legend
It doesn't say that though, nothing in there mentions the Abyss or that this is where those squirming larvae will end up. In fact nothing about these souls mentions anything about whether or not they are evil, only that they are unclaimed by the gods. They could be paragons of virtue but they still suffer the same fate as the so-called evil souls.
You are correct, but we know that soul larva are formed from evil souls in the Abyss, and that Night hags and other fiends collect, trade and devour them.
Since that is the only reference to Soul Larva I am aware of in the game, it seems to be an obvious allusion to that fate.