To me, the major theme of this story is that people have a great capacity for acting in evil ways. I think we can see this in both Conradin and his guardian, Mrs. De Ropp.
The guardian treats Conradin very badly throughout the story. She is supposed to be his parent figure yet she delights in making him feel bad. She feels triumphant whenever she denies him something that he wants.
On the other hand, Conradin is quite bad in that he wants such horrible revenge on Mrs. De Ropp. He does rituals to try to keep her toothache going and he prays for her death. When she is killed, he is happy.
So the theme to me is that people are cruel and evil.