I agree 100% with everything @foolcat said above. Another angle to this is the narration of the result. The GM, seeing the result of the damage dice, might have narrated that the cultist moved his head at the last moment, making the shot to the head only a grazing shot as a result (4 points of damage). That would work as well to keep things believable.
Or the GM might have ruled that, even though it was only 4 points of damage, the cultist was stunned because of the shot and could not riposte.
Another thing I always mention to my players is that, whatever works for them also works for the opposition. If a cultist sneak up behind a player character and puts a gun to their head, how do they feel about automatic death?