Moves a Character Makes at the Climax

Interesting dilemma in a recent plot consultation -- the protagonist (a 12 year-old in a middle grade fantasy novel) kills the evil queen, her mother, at the Climax. Now, before you react, let me explain. Turns out in the Resolution the woman she kills is not actually her mother. Whew! Still, the reveal comes too late to justify the killing as the story is written now. This age-group, heck, any age group, for the protagonist to do such a deed, the mother must be evil incarnate -- which...