Animals are described as people throughout the story.
The use of animals as characters fully acting like humans is known as anthropomorphism. It is basically a sophisticated form of personification. Personification just means describing something nonhuman as if it were human.
Anthropomorphism is demonstrated by how the animals are described as having human feelings and motivations.
This is the story of the great war that Rikki-tikki-tavi fought single-handed, through the bath-rooms of the big bungalow in Segowlee cantonment. Darzee, the tailor-bird, helped him, and Chuchundra, the musk-rat, who never comes out into the middle of the floor, but always creeps round by the wall, gave him advice; but Rikki-tikki did the real fighting.
Clearly, there is little difference between these animals and people. They are described as fighting wars, and giving advice. They have distinct personalities of cleverness, fear, and bravery. They also possess their animal characteristics and traits, or I should say their instincts.
Animals do not really have the ability to reason like humans. They cannot theorize or form sophisticated plans. Rikki does all of these things, and so do Nag and Nagaina. They are animals who think like people, at least for the story’s sake.
“I had not thought of that,'' said Nag. “I will go, but there is no need that we should hunt for Rikki-tikki afterward. I will kill the big man and his wife, and the child if I can, and come away quietly. The the bungalow will be empty, and Rikki-tikki will go.''
Nag and Nagaina develop a sophisticated plan to get rid of Rikki, and Rikki develops a plan to deal with them. He kills Nag first. He kills all of Nagaina’s eggs but one, and then uses that egg to draw her out. She tries to get him to leave her alone, saying she will go away. He doesn’t believe her.