User blog comment:Omega natsu2/Chapter 369: The Place Prayers Reach, Predictions./@comment-4449950-20140128032255/@comment-7418318-20140128132213

I'm a bit torn on this matter. On one side it would make more sense that Jellal was the instigator, but on the other hand, right before confronting OS, Jellal said he'd do this to repay the favour to Doranbolt.

But even if Doranbolt or Cobra came up with the matter, I can still see two reasons for them to bring Brain outside for his death:
 * Killing Brain in the prison could have some nasty repercussions for Doranbolt, once the next council get's elected, so it's safer to have the death occur somewhere outside. That way it's easier to deny responsibility if someone should find out. With his memory manipulation magic the risk of being exposed might be lessened, but not eliminated.
 * Cobra (whether it's his plan, or his condition for agreeing to Doranbolts plan) might have wanted all of OS to see Brain die. It would have been a bit awkward to say the least, to gather the five OS members in front of the cell and kill Brain inside.