Well, from how I understand it, when someone is tempered the primal basically imprints themselves on all the aether inside a person. Once that happens, all they can do is worship the primal and that just generates more aether for the primal to feed on. They've basically been turned into literal aether batteries. Just leaving them alive will empower a primal and hasten their return if defeated.

This is why the echo provides complete protection from tempering. Hydaelyn has imprinted herself on you so that no primal could possibly claim you. What I wonder is if tempering could be reversed if someone got blessed with the echo after the fact.