Might not be a case of him going rogue, though - Garlemald's opposition to Primals is based around them being an obstacle to their dominance of Hydaelyn, not a matter of state religion, and it's been pretty well established that they're big admirers of Allag. With that in mind, it's not really surprising that they'd happily use a method that lets them not only make Primals into a military non-issue, but harness their power for their own use as well.

It was also the Ascians that led them to Azys La in the first place, right? Or am I mis-remembering that?