In Amaurot, we learn that Lahabrea of old was apparently quite the talented individual, well-respected among his peers.
The Lahabrea we encounter throughout ARR and HW, on the other hand, is a cackling, almost cartoonish villain, arrogant to the point of making some rather dim-witted mistakes.
In contemplating why, I came up with the thought: Perhaps Lahabrea was beaten extra thoroughly with the Temper stick?
Emet-Selch mentioned that all of the Convocation who summoned Zodiark were subsequently Tempered by him, and we know from the Beast Tribe quests that over-Tempering can have deleterious affects on the afflicted, from insanity to physical mutations.
We've seen Tempered that can't seem to shut up about how great and glorious their Primal is, and others subtle enough to infiltrate the ranks of our allies.
In spite of the fact that Lahabrea DID successfully infiltrate the Scions (as Thancred), I wonder if he might be one of the former, Tempered to the point where Zodiark and his glorious return are pretty much all he can think about anymore? It could explain how a respected orator has been reduced to CHAOS, EVERMORE!!!