In Awakening, activating Lethality won't necessary mean the user will be guaranteed to hit. Since this skill only modifies the next move to be a one-hit kill, it can be countered via Dual Guard, simply be dodged like a normal attack if the user's Hit rate is low enough, and even Miracle can counter its effect.
A Lethality hit can still become a critical hit, despite the fact that the target would be killed anyway. It remains useful however as a critical hit guarantees the attack will hit the target, since critical hits cannot miss (due to Hit rate being calculated before Crit. rate) and are not countered by Dual Guards.
A Lethality hit can also trigger the target's Pavise skill to activate, despite it not having any effect since Lethality will always strike the opponent with an equal amount of damage to that of the targets remaining HP.