Great question!  While most readers assume at first glance that the title of "Poison" refers to the alleged snake that is ready to strike Harry, Dahl actually intends for the title to figuratively represent the venom of Harry's racist comments about and to Dr. Ganderbai.  For some reason, Harry hates Indians such as the doctor so much that he cannot quell his hatred even when the doctor works to save his life. Dahl uses the story to demonstrate that hate of any kind poisons an individual quickly and then begins to affect others.

The word "venom" in the poem is, of course, paramount in a comparison between these two literary works. Both authors view hatred and prejudice as a snake's poisonous strike.