Well, obviously they don't kill their own. Those ships all have crews. They didn't even kill the guy in "Bushwacked," and he was only potentially one of them. My guess would be that they can recognize people who share their reaction to the Pax chemical, and don't mind them, but it's seeing the ones who would be pacified by it is what drives them into a rage. They probably have some sort of society and order among themselves, though no one would see it since, anyone who tried to observe them would quickly be eaten.