[The agreement, even if--or, rather, especially because--he can tell Nate seems to be hesitant, means a lot; he appreciates the trust being put in him, even if it's a little terrifying. He's still not completely sure this won't all end up being a huge mistake.

But he believes this is the right thing to do, both logically and morally, and he takes a deep breath.]

We know that the Null is probably trapped under some rubble somewhere in the city, and Nick thinks it may be near to the god temples it seem to be feeling the emotions of most strongly. It's just a guess, but it's as good a reason as any to start looking in a particular place.