To send more souls to the Nine Hells, the devils seem to have stopped at nothing, committing numerous inhumane acts. But to the soul merchant, these acts were relatively normal.
Just as a lion is naturally a carnivore to survive, the racial attributes of the devils compelled them to constantly gather souls everywhere. This is the law of the multiverse, unbreakable by anyone, with no concept of right or wrong.
Elvis also told Bartlett that, considering the daily death toll on the Prime Material Plane, the devils would be fully satisfied with just a fraction of the souls. However, they couldn't naturally obtain even half of that fraction now and had to resort to some unconventional methods.
