User blog comment:B214/Chapter 504: Rift/@comment-27337151-20161001212600/@comment-5303512-20161002030632

Well Gray is still under the impression that END is the master of Tartaros and the prime demon of Zeref's books, the demons who has caused all the suffering to happen to him.

Like I said before, it is everything that has been leading to this moment, this is his legacy, and because it is his best friend that makes this all the more tougher and important to him.

also, his Demon Marks are corrupting him so he is less likely to cooperate too.