Just a bit of food for thought - we already do sacrifice the souls of people for power. Innocent people, even, and not just cultists, but you're just not thinking about it. You see, we live on Avernus willingly. It's entirely within our power to construct new cities in space or on the moons, and move our population to those places. Instead we keep our population on Avernus so they can train to be badass deathworlders who can be trained into the deadliest pure human soldiers in the galaxy. But the thing about willingly living on Avernus and inhabiting certain areas of it is that you know that by doing so you're consigning people to having their souls devoured.
Living in Azure Bay? Well, even if you've managed peace with the Sirens there's still the soul devouring Deep Ones who will eat your people by the tens of millions when they wake up from their sleep. Living in Avernus's Spine? Well, Magma Worms will sometimes attack your cities to devour millions of tasty soul treats. Oh, you want to live by the delta and the lakes in Duat? Be ready to consign a number of your citizens to having their souls eaten by Amut Crocodiles. And there's certainly more soul devouring xeno monsters out there in the regions we haven't yet explored and colonized, but we will of course colonize those places because we need to increase our Deathworlder bonus and make the Helltroopers that much more deadly.
Oh, sure, you're not exactly just handing those souls over to the xenos, but as the head of the government you've made the willing choice to continue living on a would where there are xenos who will devour the souls of your citizens and where it's practically a coin toss to determine if a baby will survive to the age that it can crawl away from the Blink Spiders, but hey, at least you can sleep soundly knowing your armies can kick the shit out of almost any foe, am I right?
But surely handing over a few cultists to the Sirens so you can ensure more of your people survive and don't get devoured by daemons is monstrous, right?
