Rain and drought become symbols of spiritual and emotional emptiness, especially for Okonkwo. Weather, particularly rain, is essential to the livelihood of the Ibo people. Without rain, the yams cannot grow, & without yams, a man cannot feed his family. Therefore he is not a man. For Okonkwo, this drought comes to reflect his inner drought, a metaphorical sterility in his heart. He feels no love for Nwoye, his first son, & he has difficulty expressing any kind of feelings for any other members of his family.
The Ibo are a polytheistic society, & they believe in particular gods for various aspects of nature. Often, they identify the source of a drought as some sort of blasphemous act that has occurred on their soil. It is much the same with Okonkwo. His blasphemy was the murder of Ikemefuna, which led to his own internal emptiness. In order to restore the rain & the land, one must make a sacrifice of some kind, a penance for sin, if you will. Okonkwo does this through his suicide. It becomes the ultimate offering for the village and the gods, atoning for the sins he visited upon the society.