User blog comment:FairyRaven/Fairy Tail Chapter 391: Gray vs. Silver, Review and Commentary/@comment-24784846-20140704210129/@comment-7418318-20140704223227

But why would Silver want to fight Gray seriously?

And I don't see why it's such a big problem that one Demon knows Demon Slaying Magic when many, maybe even all, Dragons know Dragon Slaying Magic.

If anything Demons are the most likely to have Demon Slaying Magic, since they know each other and each other's weaknesses better than a Human could and thus are more likely to be able to create a magic that targets those weaknesses.

From Galuna we know that there are Demons outside of Tartaros, so a war between different tribes of Demons in the past, in which this magic was invented is a possibility.