I currently replay FFX and got to the scene in which Lulu tells Tidus that the Sending is important. She tells him that the dead are raging so much and envy the living so that they become fiends.
I also currently play FFVIII and I got to the scene in which Squall is told (by whom I don't remember, haha .,.") that the fiends are coming from the moon. (Something with tears from the moon.. I don't quite remember that one, has been a few weeks that I saw this ^^")

So I was quite stunned that in both games there was a reason that fiends exist. In this thread, I'd like you to help me complete the list, as I don't remember if there was anything said in the other games about the fiends.

And of course: Which of these ideas do you like best? Do you think they're good or bad ideas? I mean, do you actually think that it fits the story and do you believe that this could be happening in such a world?

As for me... I loved the way Lulu told Tidus where the fiends came from. And it fits the story so perfectly.. all the fiends you beat will be sent after fighting them. That was quite nice. I told my mom a few days ago that it was a good thing to slay all those monsters and she was a bit shocked about that. But after I explained to her WHY I need to do this she was also a bit stunned. I don't think she expected a game to be so full of story xD"
After thinking about this for a while, I really began to love the whole story with the fiends being unsent humans. ^^