Well, the Doctor is a dishonorable character from start to finish. Note how when he first hears about the scorpion sting he doesn't bother to go because he is only interested in the wealth that he can gain from patients. However, now that Kino has found this great pearl, he wastes no time in coming to them and deliberately manipulates their fear to convince them that their son needs more treatment:
He said, "Sometimes, my friend, the scorpion sting has a curious effect. There will be apparent improvement, and then without warning - pouf!" He pursed his lips and made a little explosion to show how quick it could be, and he shifted his small black doctor's bag about so that the light of the lamp fell upon it, for he knew that Kino's race love the tools of any craft and trust them.
The Doctor is thus extremely unethical in the way that he manipulates Kino, trying to convince him that his son needs further treatment when he obviously doesn't, to try and exploit Kino and gain the wealth of the pearl that Kino himself has found. Unfortunately, however, the Doctor is not the only character who responds in this way, as Kino finds that having wealth is far more dangerous and complicated than lacking it.