"At this point, I wonder if we shouldn't just let him get eaten as a statement to the rest who like to run off..." Childermass says very much out loud as he watches Rin rush off into the tree. That said, he's still hoping it's only discussion waiting for them in there.

But since he's being bid to go with, that's what he'll do. He turns to go, only looking back one more time to tell Lambert, "Try not to find more trouble while you're out here." Which is more or less 'be careful'.