In my opinion, there are various themes that you might find in this story. I will discuss what I think is the most important theme.
I think that this story is about how far evil people will go to accomplish their goals and how "civilized" society is helpless in the face of such tactics. Evil overcomes good. You can see that in this story because the civilized people are completely unable to do anything to the Adams twins. They only punish the twins by reverting to savagery themselves.
Even after they do this, the author implies that the witnesses (who tried to act in a civilized way) will be in mortal danger from the surviving twin.
hmmm,...same as the pohnpei397