"The stars will rain from the sky before Mab fulfills not her word." - Mab, Changes.
The above declaration was actually prophecy. It refers to the BAT book Stars and Stones, when stars will rain from the sky. Once they do, Mab will break her word. However, it is impossible for Mab to break her word -- unless she is nemfected. This is what Harry's true purpose is - Mab wants him as Winter Knight because the Fae Queens are vulnerable to their own power (their own knight) and Harry is the best chance she has at grooming a knight that will be able to kill her. At some point, Mab will reveal to Harry that she is fighting nemfection and ask Harry, as a favor, that when she finally succumbs for him to kill her. Mab failing to refuse her word will be the sign that Harry needs and Harry will successfully kill her at the end of Stars and Stones.
Mab's death will destroy the defense of the Gates, allowing the Outsiders to enter the world for the final book of the trilogy - Empty Night. Further, by killing Mab as a favor, Harry will fulfill the 3rd request from his earlier deal with Mab, and obligation to Mab will cease, including Harry's promise in Changes to become the WK. By that point, Harry will have become accustomed to the power of Winter and in control of the mantle and losing that power will handicap him significantly, and the third book will be a rush for him to learn the true power of a Starborn so that the Outsiders can be repelled and a new defenders of the Gates can be selected.
Based on the above, my emotion when Mab dies will be a drop of sadness in a pool of fear - because it means the all of existence in the DV will be threatened.