If you do kill them, I'd make it abundantly clear that the souls they have been adding the whole time are the reason.
Here's how I'd do it:
Like, the demon makes some hand motions, a cloud of smoke with movement inside appears over it's head - Perception check, oh, you see the faces of the last creatures you killed, writhing in agony! and their forms wither in agony as they are sucked into the ball of energy forming in it's hands. It casts, and as the spell blasts over the whole party, wizard, make an Arcana check - yes, this spell is one you recognize but way more powerful than usual.
Or, the Fighter attacks, and you can see each sword blade is interrupted by a form that is floating around the demon in the air. Yes, each one is a soul you recognize. And every single thing they do is absorbed by that swirling mass of tormented souls without touching the demon, ablating off, soul by soul, of the dead they created. They can tell the shell is reducing in power and size, but it's still so thick, their spells are dwindling, and two of them are making Death saves.
Arcana check! Oh, it's expending those souls to fuel it's power. Intelligence check! Oh, the souls we recognize are in the reverse order of the beings they've been slaying.
Stuff like that.
Rub their noses in it. Kindly.