Well, anyways, we don't know the story about the Devil Slaying magic, so don't be so quick to judge it. Though it probably is as simple, but all we know about it that Silvers is the first in the series to wield such a power. Where it came from, no one knows. We know about the Dragon slayers, Enel slayers are just a magic picked up from books or shit like that, and none of these slaying magics have tattoos on their wielders, except guild crests. So there's actually a chance that silvers got that crest from an actual devil. Anyway it could be a curse mark or a power mark or whatever, but we know for sure that it has juice in it.
But you guys are probably right, no thought was put in that and it's just a random shortcut for powering up Gray.