Carrie's agency is almost nonexistent for the first two acts — she is acted upon by everyone: classmates, her mother, the prank plotters. She rarely makes meaningful choices; things happen TO her. Agency ticks up slightly when she decides to go to prom and demonstrates telekinesis to Margaret, but even the prom itself is someone else's plan (Sue's kindness, Tommy's invitation). The blood dump is the hinge: agency spikes violently as Carrie CHOOSES to use her power, but control plummets because she cannot stop. The destruction is all agency and no control — the inverse of her entire life before it. By the end, both lines converge at zero.