What was the relationship between Rikki and the two cobras in the story "Rikki-Tikki-Tavi?"
Rikki and the cobras are mortal enemies. Although Nag and Nagaina are the first live cobras Rikki has ever met, his mother taught him that his purpose in life as an adult mongoose would be "to fight and eat snakes." Nag understands that fact about mongooses, and it scares him. In fact, he "knew that mongooses in the garden meant death sooner or later for him and his family." Therefore, at their first meeting, Nag and Nagaina attempt to kill Rikki; Nag keeps him engaged in conversation as Nagaina sneaks up on him from behind. However, Darzee warns Rikki, and he jumps out of the way just in time.
Thereafter, the cobras seek to kill Rikki and he seeks to kill them. Nag at first is not convinced they need to kill the people, but Nagaina explains that if they kill the man, the mongoose will leave. So the snakes' desire to be rid of Rikki, because they know he will eventually kill them, is what makes them go after the humans. Rikki succeeds, with the help of the man, in killing Nag, and then he turns his attention to the cobra eggs and to Nagaina. Meanwhile, Nagaina continues her effort to kill the humans by going after Teddy on the veranda. Rikki breaks that up, draws Nagaina away, and pursues her down her hole, where he kills her. Both the snakes and Rikki want to kill each other, but the snakes fear Rikki more than he fears them.