User blog comment:FairyRaven/Fairy Tail Chapter 423: Do It All for Love, Review and Commentary/@comment-103.14.62.54-20150309110452/@comment-9595671-20150309140109

Its even possible after what happened to Grimoire Heart on Tenrou Island (provided if he even knows if Hades' survived or not) he had to go on the run and try to find somewhere the council wouldn't notice.

Even if he's in a Legal Guild, he would have still been a Dark Wizard from his crimes so he was forced into doing things to keep them from finding out. Training could have easily given him away, so group attacks like this were the best chance he had to do anything.