Pelayo and Elisenda initially treat him like an animal, locking him up with the hens in the coop. When their child gets better, they decide to send the old man off on a raft with small provisions. Even this is only a half-hearted attempt at kindness. They abandon this idea when they determine that they can profit from the old man. The others in the neighborhood treat the old man like a circus attraction. Even though one theory is that the old man is an angel, he is so disheveled that no one gives him any amount of reverence. Pelayo and Elisenda only see the old man as a source of money. No one considers the old man's feelings. Part of this is because no one can understand him. It is their lack of understanding, and lack of willingness to understand, that makes them treat him so terribly. Unable to get the old man to play along, they even burn him with an iron, mistaking his cries for rage instead of pain. Despite the riches that the old man brings to the couple, they still treat him like an annoyance.
This story is an example of magical realism, the blending of the mundane with the miraculous. There is never any closure on what the man is: a human/bird hybrid, and angel, an illusion of imagination, a metaphor, and so on. This uncertainty about what he is, part of the style of the story, is also part of the theme in the story. No one knows for sure what he is. They only speculate, coming up with different theories, as if he is simply a lifeless object to be studied. Whether he is magical/spiritual or simply an annoying animal becomes irrelevant to the townspeople. They are only interested in using him for their needs and then categorizing him. Categorization, here, trumps understanding. They would rather label than understand the old man.