Complex sentence: Cassie reveals that it was she who allowed the blue box to be taken, and Ax now nurses a hatred born of betrayal for her; for her and for the others for just forgiving her and acting "childishly".
Split into simpler sentences: Cassie reveals that it was she who allowed the blue box to be taken, and Ax now nurses a hatred born of betrayal for her. For her and for the others for just forgiving her and acting "childishly".