Ok. Something fundamentally not being understood ITT is that, per Emet dialog at the end of 5.0, they were being tempered by Zodiark (not that you really need that because the walk cutscene with Venat clearly shows us this imo). Venat also became a primal via volunteers. So it's not even just Venat and they had no other option in the face of a primal who was demanding more sacrifices and tempering.

Also per the lore Venat didn't even know exactly what her attack would render in great specificity and it didn't kill everyone.