User blog comment:FairyRaven/Fairy Tail Chapter 325: Unity, Review and Commentary/@comment-112.205.41.120-20130330011250/@comment-7418318-20130330071405

I think that angel stuff was another hint at Yukino's missing sister actually being Angel from Oracion Seis.

As for shadowmagic being evil, I'm an advocate of magic being an extention of the mage and thus by itself magic can't be good or evil. Thus I believe that just like you can use light magic to kill innocents, you can also use dark magic to punish only the guilty and thus save lives.