Based off of evidence gathered primarily from chapter 2′s pacifist and snowgrave route, this theory states that Noelle accidentally froze Dess to death and Asgore was fired from the force because of it. Throughout each mention of Dess, it is always in past tense. While it is possible that she might of moved away, we are given indication or mention of Dess’s current situation. It is instead through Berdly’s anecdote about how Noelle broke down after being asked to spell December do we learn of Dess’s fate. We already know that Dess is short for December from earlier when we spelt it out with Noelle. Furthermore, we know it can’t be from stress. While Noelle is presented as an anxious individual, she was still able reach the finals and December is not a particularly hard word to spell. Nor can it be due to simply missing Dess in the same way that Kris does with Asriel. Based off of Noelle’s and Birdie’s age as children in the flashback along with the assumption that Dess is around Asriel’s age than Dess should of still been present in her life. So ruling those out and based off the intensity of Noelle’s reaction, Dess must of died sometime before the spelling bee. As to the cause of Dess’s death, I do not believe that Asgore was in anyway directly responsible for it, accident or not. Rudy still holds Asgore in good regards nor is he in prison. Considering that Noelle’s mother is mayor (who we’ll call Carol here) as seen through the similiarities between our various characters mention the two, then if Asgore was directly responsible for Dess’s death than he’ll likely be in prison. So no, Asgore has nothing to do with Dess’s death. It was Noelle who was the one who killed Dess due to stress triggering her ice magic. When we check out the frozen aisle section at Sans’s grocery store, we learn that Noelle would often stare blankly into the frozen aisle section. Then, if you play the Snowgrave route, the several instances in which Noelle uses her ice magic on a strong scale such as with the puzzles, the shopkeeper and finally Berdly, it always triggers due to some sort of stress and involves her blacking out during it. While we know that magic isn’t as prevalent among monsters as it is Undertale, we know it still has to exist in some manner due to Catti mentioning how she was doing research on occult stuff and magic at the diner. Noelle’s magic in the light world likely isn’t as strong as it is in the Dark World but it still would of been enough to kill someone especially during winter. Noelle likely killed Dess when a stressful situation triggered her ice magic. The exact cause is difficult to assume. However, based of Dess’s desire to leave for the big city and how their mother can be hard, I believe that Dess tried running away and Noelle ended up tagging along. The two quickly became lost during which the fear and stress that Noelle would of been experiencing triggered her ice magic and blacking out. Carol, not knowing where her daughters are and with an unexpected snowstorm, orders Chief Asgore to send a search party for them only to instead find Dess’s body and Noelle unconscious in the snow. Carol, blaming Asgore for failing to find Dess in time, forces the Police to remove him leading to a rookie Undyne to suddenly being given the position of Chief. Asgore also blaming himself would end up cutting ties with Rudy unable to confront him due to his guilt and is likely one of the factors which lead to his and Toriel’s divorce. Noelle meanwhile would of have no recollection of the event instead believing everyone else that Dess froze to death thus also developing a trigger for extreme cold. So in short, Noelle accidentally killed her sister with ice magic and Asgore instead takes the blame for failing to find them in time.