At this point, it’s somewhat unclear why Lettie won’t tell the narrator what the varmints are. It’s possible that she’s trying to protect him from this knowledge, but if this is the case, it means that the narrator cannot effectively calculate what risks he’s taking as he follows Lettie. It’s also significant that Lettie specifies that they want to send Ursula
home, not just away. It’s not enough to make Ursula leave the narrator’s family alone—Lettie has to do what’s best for Ursula and for all humans, as she has a responsibility to help others.