Russell’s plan has completely fallen apart, and without his tablet his options are running out. If he can’t get the sorcerer body he’s always wanted, he can still kill off all the witnesses to his crimes before they are freed.
Russell finds out the person he’s been fighting has actually been Isabelle. Since he’s seen that she is a vastly more powerful and knowledgeable sorcerer than Chris, Russell decides that SHE would make a better body for him.