Walter: Don't you see what I've done? I crept over in the night, and I stole their child. If we don't return him, they'll figure it out, and they'll come after him, after us. I know because that's what I would do.
Elizabeth: So you would sacrifice one for the other... this little girl Olivia for Peter?
Walter: No. But for thousands of others, or millions... it would have to be considered.