Seymour DID turn into a feind when he died, didn't he? Auron didn't go bad because he was still furfilling his promise to Jecht to bring Tidus back and I think there was something to do with his going to Zanarkand in there, too. On Maester Mika........ HES SO OLD, HE LOOKS LIKE A FEIND TO ME! He turned into a feind, but in human form, I think- he was evil, wasn't he, so....

So, I guess if you die in Spira you do become a feind, but Aurons was a special case.