User blog comment:FairyRaven/Fairy Tail Chapter 375: The Supernatural, Review and Commentary/@comment-7418318-20140314102909

I think Franmalth is only using his own magic, because it wants to absorb Natsu, rather than just destroying him. It's so obsessed with cost and profit that to it must think that just annihilating Natsu would be a big waste of potential ressources. Seeing that it already absorbed Lighting-Flame magic, Franmalth was already proofen at least partially right.

And seeing how chanceless Natsu was so far, from his own point of view Franmalth has nothing to lose by dragging the fight out.

I wonder how exactly the magic absorbing worked. Will he be able to just fire out the amount of magic he absorbed from the attack, or is he now able to use it at will and for the rest of his life?

Since I doubt that magic itself has a soul, I guess the former is more likely, much like Natsu was able to use God Slaying Magic temporarily back on Tenrou, but it didn't stick.