[quote=@floodtalon]
[@Lugia]
How does David know Chen has a weapon? Chen hasn't even mentioned his knife yet.
[/quote]

You didn't have to. David already assumed Chen had ill intent, and from what he knows about humans (in his six months from being on the surface), those with ill intent tend to carry weapons. He doesn't know it's a knife, yet, but think about it... pretty much ANY weapon is deadly to a monster. Heck, a TOY is a deadly weapon underground.