While Billy’s specific plan has been somewhat vague up this point, the details here of him setting up for it make it fairly clear: he intends to dump pig’s blood on the Prom King and Queen, who should end up being Tommy and Carrie if Chris’s rigging comes through. It is notable that Billy doesn’t even know Carrie; he’s simply doing this to satiate his and Chris’s desire for cruelty. In contrast, the detail from Sue’s autobiography shows that, despite the pain and anger she holds, Carrie is a fundamentally compassionate person, even to those who are abusive to her.