Emma wouldn't want to kill all the demons since she knows there are nice ones, but its for the best because even "nice" demons need to eat humans to retain their form and intelligence
There's some speculation that Musica is either a human-demon hybrid or a human in disguise since it was pointed by Sung-something that she Never ate humans.
just catching up here but why how can musica and the other demon dude retain their human traits? i know the dude was erasing their tracks from the death squad, so maybe they ate the corpses of the kids? are they sustained via religion or what?