The Saw 2 ending shows that Amanda was never a simple victim, because she survived her own test, became John Kramer's helper and set up the game for Eric. Here I give my reading of what the final scenes mean, with full spoilers.
What happens. Eric is knocked out and shackled in the house's bathroom, the same room seen earlier with bodies in it, and the setup echoes the chained bathroom of Saw (2004). A recording shows that Amanda is John's accomplice. She walks out, seals the door and leaves Eric to die, while John smiles as he hears Eric's screams.
Was Amanda acting? In my opinion, yes, in the sense that she was playing a captive while working for John, though the film does not tell us how much of her fear was real. The tape says she survived her own test and became his helper, which reframes her scenes with Daniel. She helps him, and it could be argued that a Jigsaw apprentice would want his son alive, since the real target is Eric.
Why the factory scenes matter. Eric's colleague realizes that the footage on the monitors was recorded earlier and not live, so what Eric watched was never a real time view of the house. Eric's mistake is that he stops talking and starts hitting, which is exactly what the game was built to provoke. A stubborn detective was always going to leave the table.
Where it leads. The last scene sets up Amanda as the apprentice who will carry on the work, and she becomes a major figure in Saw III, where Eric gets out of the bathroom trap. If you want the wider picture, the Saw timeline explained page puts the reveal in context, and Saw II is third in chronological order because Saw X is set between the first two films.
