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 the queen is though not necessarily shown enough throughout the story as it's written now -- and even then, I believe it is a tough sell for middle-grads readers, or at least their gatekeeper -- parents, teacher, etc.
Not even Luke Skywalker is able in the end to kill his own father -- Darth Vader -- in the Star Wars films.
The archetype of the Mother needs to stay pure. The woman she has become can be hated -- yes? -- but...
The Climax is the crowning glory of the story. The reader has been reading for pages and pages. This is the scene they will likely remember. To have such controversy at that moment can work in adult fiction, but in middle grade fiction...