For those who have lingered long enough in the Outer Layer, selling their soul to a demon is a cliché and terrible scam. Some fools might think being reborn as a Bartez after death doesn't seem too bad, at least you would have eternal life, plus a predictable promotion mechanism, a sound payment system, and even paid leave benefits...
But who said you could directly become a demon?
The souls that travel to Hell are first treated as 'fuel', subjected to unimaginable torture and torment, their energy extracted to maintain the operation of the Nine Hells, until not a trace of humanity remains in the empty shell of the soul. This process could be brief or it might last for years, and if you have a feud with a high-ranking demon, you might even be perpetually forgotten on the grill, until the end of time.