Queen talked into a magic mirror, that she chose based upon it's agreeableness, and every day asking if someone was more beautiful than her, mirror appeased her and said yes there is and it so happens to be the step daughter you hate. I should kill her, shouldn't I magic mirror. Yeah, people in fairy tales kill their step daughters all the time, no problem there.

Queen does it, and the kingdom is then mad at the mirror, and king rules that all mirrors need to be destroyed.