User blog comment:EvanDude345rv5/Fairy Tail 353 : Nine Ghost Gate/@comment-3424593-20130928223035/@comment-7418318-20131001075130

Brain never even mentioned Zeref. So while there is nothing to contradict your theory that Brain was a Zeref follower, there is nothing to support it either.

To me it seemed that Brain/Zero didn't follow anything other than his own desires and he only joined the baram alliance out of convenience.

The same probably applies to him teaching Jellal. I think he either did it to fulfil a deal, or because he thought teaching Jellal would bring destruction to the world.