One possible theme of the story is the inadequacy of merely human justice. Despite a lengthy trial, and despite the killing of one of the twins, justice has still not been served. Neither a formal legal trial in a court of law nor an act of vigilantism has been able to provide satisfaction for anyone in this whole sorry business.

Mrs. Salmon won't be able to sleep soundly in her bed for one of two reasons. She may have been the one who pushed the twin into the oncoming traffic, humiliated as she was by her cross-examination on the witness stand, and yet now finds herself wracked by guilt. Or, she could be scared stiff that the killer or his twin brother will come to take revenge on her. Whatever the reason for Mrs. Salmon's imminent unease, there's no doubt that she, like the original murder victim, has been short-changed and victimized by human justice, however broadly one defines it.

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.