User blog comment:RoyalGuard-Elite/Confusion/@comment-7418318-20131003081004

Possessing Devil Slayer Magic does not automatically make him good.

There have been good mages who used Darkness Magic and there have been questionable ones who used Light Magic.

He might have learned the magic before turning evil, or as a way to be able to compete with the Demons in Tartarus (if Succubus Eye had a Demon in their ranks, Tartarus might have them too).

Or he might have aquired that magic to be able to beat Zeref. You can't conquer the world for yourself if it's destroyed by Zeref after all.