User blog comment:Miskos3/Chapter 478: Stealth, Review/@comment-1359322-20160319140344/@comment-7418318-20160319152724

I think the chapter makes it abundantly clear that Jacob's only playing with them.

He could just kill them at any moment by disappearing and slicing their throats or stabbing them through the heart, but instead he wants to torture them, probably to hurt Mavis through them and make her spill the location of her real body.

But after getting deceived by Natsu and Lucy he seems all out of playfulness and seriously pissed off.