The way Eren speaks of it is very telling. Not that Billy disagrees with him that an existence like that would be miserable, but there's something about the usage of the word livestock that stands out to Billy. It almost makes it seem like Eren has a particular dislike for it. Coupled with the way he's saying they can be killed-- Has he tried? Did he set up some sort of movement for it?

He's partially speculating because he's just nosy, but he's also figuring it means Eren might be able to do something about the Darkwalker. Or at least try. It would certainly help them here.

Billy doesn't bring it up just yet though. It's the sort of thought he has to ruminate on first, especially when he never says stuff carelessly. Instead he speaks up when he's addressed once more with that question.

"No titans, though I imagine you already understood as much," he first says. "There are no real things there that anyone would classify as a monster, if that makes sense. The only real monsters are fellow men."

There's a slight pause, and his eyebrows draw closer together as he adds: "Or so I believed, either way. I went to the Arctic as part of my work for the Discovery Service, and we-- we did run into a monster there. It seemed like a bear at first, but it was more than that."