I think if Natsu is E.N.D. than he can't be the big hero that beats the final boss like usual if he is the final boss himself, so maybe him stealing the glory and taking down some of the 9 demons is Hiro way of giving him some glory because he is not going to get the final boss like always.
If it is not Natsu, who is going to fight E.N.D., I think it should be Jellal, Lexus, and maybe Gajeel, former enemies that wrapped in darkness that Natsu saved from darkness, and now they are returning the favor and saving Natsu from darkness. Of course Hiro could really surprise me and let it be Lucy that stops evil Natsu.