I thought they explained that pretty clearly in VI.
Spoiler:
He didn't want to continue and he knew that if Amanda were alive, he'd have to keep playing along.
Hmm, really?
Spoiler:
I interpreted as him wanting to do things his own way after Jigsaw was dead and with her around that wouldn't be possible. He set her up to fail in Jigsaw's eyes but it just ended up that she got caught up in that crossfire so it worked out for him. It seemed like he never had any intention of stopping. He enjoys it.
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