User blog comment:IamJakuhoRaikoben/Chapter 379: Tartarus Arc, Part 3: Underworld King, Review./@comment-5598380-20140404081256/@comment-7418318-20140404113450

He was always set up as a character who looked too deep into the darkness and became corrupted when something started to look back, so it's not a big surprise for him to return to his former character after seeing that all his diving into darkness not only destroyed him, but would never have brought him to his goal in the first place.

And while the warning and apology might make Fairy Tail forgive him, it won't be enough to balance all the sins he committed and made others commit. While he might have been corrupted, he was still acting mostly or maybe even entirely of his own will.