So going off Haumea saying something about human perception factoring into adolla last chapter, maybe why some of the infernals could talk is because their doppelganger still has similar a mindset? The very first infernal we saw talking I think was that criminal guy back in the first few chapters and he was already a killer and crazy so people's perception of him was the same so as his doppelganger took over he had the same demeanor and wanted to kill people. Or maybe the perception has to be strong enough, oh shit is that why some turn into demons because of how strong people perceived them like Konro's demon and now Benimaru's master? Jesus idk