An evil troll (called the "devil")[2] has made a magic mirror that distorts the appearance of everything it reflects. It fails to reflect the good and beautiful aspects of people and things, while magnifying their bad and ugly aspects. The devil, who teaches a "devil school," took the mirror and his pupils throughout the world, delighting in using it to distort everyone and everything; the mirror makes the loveliest landscapes look like "boiled spinach." They tried to carry the mirror into Heaven with the idea of making fools of the angels and God, but the higher they lifted it, the more the mirror grinned and shook with delight, and it slipped from their grasp and fell back to earth, shattering into millions of pieces. These splinters — some no larger than a grain of sand — were blown around and get into people's hearts and eyes, freezing their hearts like blocks of ice and making their eyes like the troll-mirror itself, seeing only the bad and ugly in people and things. On a pleasant summer day, splinters of the troll-mirror get into Kai's heart and eyes while he and Gerda are looking at a picture book in their window-box garden. Kai becomes cruel and aggressive. He destroys their window-box garden, he makes fun of his grandmother, and he no longer cares about Gerda, since all of them now appear bad and ugly to him. The only beautiful and perfect things to him now are the tiny snowflakes that he sees through a magnifying glass.---------
Now replace Kai with Killian and you basically got the idea. In the original Snow Queen story, there is a mirror that shatters and effects the people who come into contact with said mirror and it makes them cruel, bad people. Our theory is Killian gets effected and then Emma has to fight to get him back to HER Killian!