It's worth noting that for just a moment, even after Arthur has turned, the other man's gun is focused not on him but on the doorway where the creatures are retreating. That doesn't last though, and it's the kind of delay that could have cost his life, so he's frowning by the time his aim shifts to the other man a moment later.

The man that's behind the shotgun is the nearly-uniform shades of dust brown that says he's been traveling the desert a long time, although the bike he's on is slightly better off than he is. Right foot on the ground, he's still astride the well-packed vehicle, and the amount of supplies he's carrying speaks of long travel, too, so it's unlikely he's from the immediate area.