User blog comment:IamJakuhoRaikoben/Chapter 424: Avatar, Review./@comment-72.159.148.11-20150317162055

"Before Gray and Natsu get a chance to attack, Mard utilizes his Memento Mori and claims victory. However Gray blocks the attack, which causes half of his body to be demonized, seemingly falling defeated." <- from this wiki on grays page Maybe that could be a cause to why hes evil.