~Sigh~
The big bad just walks away...
Why? Why leave anyone alive...
Author your story is shit. The demons are dead set of destroying the world. They wipe out entire countries, and use their souls to power their magics. So why in hell would they leave free fuel walking around when they absolutely had the means to slaughter every man, woman and child left in the rubble of the broken capital?
I don't buy the enjoyment of prolonged suffering BS, nor that the humans are worth more alive. The only reason there are survivors is because your protagonist was in this location and you couldn't justify killing them without killing him.
So the big bad just walks away.
What utter bullshit.