User blog comment:FairyRaven/Fairy Tail Chapter 365: Fairy in the Jail!, Review and Commentary/@comment-7397924-20131229083136/@comment-7418318-20131229084456

I wouldn't call that a betrayal, since he never had any obligation or made any promise to help them.

Besides, after what Cobra told them about Brain considering them disposable tools I doubt they'd hold Jellal's attempt to destroy Nirvana against him. The only OS member who fought against Jellal is midnight and he's not even fighting Jellal now.