The peeps of Kilika would have turned into monsters if they were not sent. They did not accept death AT ALL!
Well, the way I understood it, their "not accepting" death wasn't so much of a conscious choice as it was a result of the violent and horrific manner in which they died. I think that people who die in this manner, as a result of tragedy (before their natural time, so to speak) are the ones which need to be sent. With Sin killing people left and right, it would then make sense why so many fiends existed.