User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-114.108.214.205-20151209150638/@comment-7418318-20151209152915

Strongest without a doubt, but why the most evil?

Zero wanted to destroy everything that has form just for the hell of it and killed because he wanted to kill and liked to kill.

Zeref on the other hand has only intentionally killed Hades throughout the current series and that seemed more like delivering justice to a criminal than an evil act. Sure it fell into a morally grey zone, but it certainly was not pure evil.

Even Jose seemed more evil than Zeref and he technically wasn't even a Dark Mage (as in Dark Guild member against the law and all that... of course he used Darkness magic, but that's not what we're talking about here).