The doctorKino suspected the doctor poisned Coyotito. was the suspicion justified?
Yes, in the beginning of the story Juana wants the doctor, and the doctor does not come. They go to the doctor instead. We learn that not only is the doctor a bad doctor, he is a also a bad person. He is described this way:
They knew his ignorance, his cruelty, his avarice, hisappetites, his sins. They knew his clumsy abortions and the little brown pennies he gavesparingly for alms. They had seen his corpses go into the church.
The doctor is a symptom, you could say, of capitalism run rampant.
Putting aside his oath to heal, the doctor instead chooses to use his position - and abuse the trust vested in that position - to swindle and essentially cheat people out of what little money they have.
Yes. Absolutely. If you read the text carefully the omniscient narrator clearly tells us that this is what the doctor does, and that this is something he does on a regular basis, to ensure that his patients need further treatment and therefore he can drain them of more money.
The doctor is one of the least attractive characters in the entire work. This is ironic, because we normally think of doctors as among the most admirable of human beings. Steinbeck was obviously playing here on our traditional, conventional expectations.