What I believe happened to the sinners in fraud:

So, we know that the mirror reaper is the only one of its kind, but I personally believe that there were more (it said it was the only one to be seen). It's mentioned in geryon's terminal that it hunted down sinners in fraud to (presumably) kill them to free them from their torment. This, paired with how there is only one mirror reaper, makes a lot of sense, even if the theory is wrong, geryon still hunted down and killed the sinners. This makes fraud the least populated layer in the game