was no snake at all is an anti-climax. 

The theme in this story is fear and paranoia. More specifically, it is about fear bred from a limited understanding of what we know. Both Timber Woods and Harry Pope have a rudimentary knowledge of the dangers a krait poses. Their understanding is based on what they have heard and on rumor. None of them has ever been involved in a direct confrontation with such a poisonous snake. Because of this, they are both desperately afraid and in a quandary about how to deal with this supposed threat. They, therefore, need the assistance of an expert who is presented in the form of the knowledgeable and kind Dr. Ganderbai who is more than willing to sacrifice his time and resources to help a fellow human whom, he believes, is in desperate need.

This act of kindness is, however, lost on our much-relieved victim, Harry Pope. He allows his prejudice to come to the fore when he expresses his vehemence on the undeserving doctor. Just as much as his knowledge about exactly how to deal with the supposed snake was limited, so is his understanding of the doctor. One may assume, when one takes into account their names, that they are of a different race to the doctor who is clearly Indian. Harry's prejudice is borne of a supercilious attitude. His outburst is completely unwarranted and hurtful.  

The story is told from a first person, limited perspective. Timber Woods is the narrator and he reports in the minutest detail all the aspects relating to Harry's unfortunate position.  All he can comment on is what he witnesses and, obviously, his own feelings. Timber is not critical or judgmental. He, however, does share Harry's anxiety. He does apologize on his friend's behalf after Harry's vile tirade. His judgment of Harry is clearly based on his concern for him, for he says that his outburst was as a result of the enormous stress that he had suffered throughout his ordeal.

The tone of a story refers to the writer's attitude towards a character, place or development. Roald Dahl's tone is clearly satirical. The fact that he chose such archetypal names as Timber Woods (quite humorous), Harry Pope and Dr. Ganderbai, clearly supports this. Furthermore, in depicting what is actually a harmless situation in such a serious and dramatic manner, Dahl emphasizes his scornful attitude. He clearly makes fun of the irrational fear that others, who deem themselves superior, hold of things they obviously do not understand. This fear is expressed through prejudice - it is a protective mechanism.

Theme: The theme in Roald Dahl's short story "Poison" is basically the conflict between reason and self-imposed neurosis.  Harry represents the typical Westernized man whose tendency is to over-work himself over the small stuff, or over nothing at all. This is a clear call to think about the Western culture: Perhaps Roald Dahl views the Westernized world as one which is too nervous, too neurotic, and too traumatized by first world problems as it is. 

On the other hand, Dr. Ganderbrai, an Indian, represents the Eastern culture; one which is known for its wiser, older traditions. People like Dr. Ganderbrai, the "Indian doctor", are needed to put matters into their true perspective. After all, once Harry made the spectacle that he made, Ganderbrai's opinion was simple. 

"All he needs is a good holiday" 

Climax: Certainly the climax of the story was the discovery that there was no snake at at on top of Harry's belly. That shocking moment brings the rest of the plot to a slow but steady end. 

Tone: The tone of the story starts out quite agitated and serious. However, as the story progresses, after Dr. Ganderbrai enters the story, and after the climax the entire story turns ironic. Hence, the tone is comical and ironic, sarcastic and critical at most. 

The point of view is a first person omniscient, since Timber is doing all the talking as an anecdotal tale. He is not omniscient, though, because he gets to find out what is really going on as the situation unveils, even though he is telling us about it in past tense.  

This short story is different from many others in that the main dilemma is saving a patient from the mortal bite of a poisonous snake which finally really isn't there at all!  Dalh deftly builts up both suspense and angst around a "problem" and then pops the bubble, so to speak, at the very end. In such a way, it's not only the patient who feels a bit foolish but the reader as well....

Your question is in fact several thrown into one. In chronological order, the tone is serious and heavy, then detached at the end; the point of view is third person limited, interjected with a lot of dialogue; the climax, is when the doctor pulls back the cover only to find the snake is not really there at all. One theme is that many problems are not really as big as they seem and are simply a mental battle to be dealt with. Another statement could be formulated around the subject of credibility and credulity; for instance: 'It is not wise to believe everything you see and hear without first checking out the facts.'