User blog comment:FairyRaven/Fairy Tail Chapter 366: 1000 Souls, Review and Commentary/@comment-5598380-20140109212812/@comment-7418318-20140109220356

The soul thing depends on which translation you read (from your comment I'd guess you read Mangastream)... unless both are wrong.

Though I doubt that it's a price for Natsu to pay. More like a price for mankind to pay.

Or maybe... if he'd turn Natsu into an obedient Demon-slave, he could let Natsu "work off his debt" by killing 2000 innocents before granting him death. Of course he'd had to leave Natsu somehow able to regret the death he's causing, while still being unable to prevent himself from doing so, or death wouldn't be a welcome release. That'd actually be an interesting concept I think, even if it's a bit disturbing.