Ant, honestly I think you're misinterpreting what actually happened.
Vitiate overwhelmed Revan's defenses, and Vitiate kept electrocuting him with more and more Lightning as Revan (who at this point is defenseless) was convulsing in agony. This stops when T3 steps in to cook Vitiate. Vitiate not ashing defenseless Revan is an inconsistency, just like Sidious not ashing a defenseless Windu, Valkorion not ashing Marr, etc.