Angier thought sometimes, when he could think clearly about Borden at all, that maybe he wouldn't take things as far as he was planning to if it weren't for the girl. They had both lost their wives, both crippled the other for life, and, though Borden had stolen Olivia, Angier's own obsession had allowed her to be taken. Perhaps in a slightly different life he could have called things even then, once he'd finally bested Borden at his own trick. He could have kept the area below stage firmly locked so no one else would ever know the secret to Tesla's machine, and been content knowing that Borden would spend the rest of his life being as obsessed with the working of The Real Transported Man as Angier himself had been with the original.
But Borden had also had a daughter, the child that Angier and Julia had never had the chance to have. The possible child that Borden had stolen from him along with Julia's life. It was one last debt that Borden had to repay, and there was only one way Angier could think of to take Jess as his own daughter.
The pledge and the turn were already in motion. And when at last Borden's obsession got the better of him, when Angier spied him slipping closer and closer to the stage in the false beards and wigs he wore every night, then at last he would go through with the prestige.