User blog comment:IamJakuhoRaikoben/Chapter 360: The White Inheritance, Review./@comment-2214433-20131115125846/@comment-7418318-20131115152051

Who sais that Demons are evil incarnate? Considering the Galuna Island inhabitants Demons might very well be just another normal race with good, bad and neutral people depending on their upbringing and the events that formed their personalities, just like Humans, Exceed, Dragons or the Frog people.

Of course since they were either created or somehow selected and changed by Zeref at a time when he was still evil, it's unlikely that the Nine Demon Gates are really "good people who fell in with the wrong crowd", but I doubt they are negative emotions made manifest either.