How does Rikki-tikki threaten the snakes in "Rikki-Tikki-Tavi"?

Rikki-tikki is a natural enemy of snakes because he is a mongoose, an animal known for hunting and killing serpents. Rikki-tikki prowls throughout the garden and the bungalow in search of the snakes. Rikki-tikki also comes from a family of snake killers and was raised on a diet of cobras. He is a threat to their existence, and also a threat to Nagiana's unhatched eggs. Although cobras have a poisonous bite, mongooses are immune to cobra venom, making them a deadly predator. Karait, Nag, and Nagaina all fall victim to Rikki-tikki by the end of the story. Rikki-tikki also threatens the cobras' position of authority in the garden. Before Rikki-tikki arrives, Nag and Nagiana roam freely throughout the garden threatening other animals like the tailorbird and the muskrat. Following Rikki-tikki's arrival, the cobras are no longer the highest members of the food chain in the bungalow and the surrounding area. Rikki-tikki ends up ridding the compound of snakes and assumes the role of authority figure of the bungalow.

In general, Rikki-tikki is a threat because he is in the same habitat as Nag and Nagaina. A mongoose is a predatory animal, so Rikki-tikki is capable of efficiently killing another animal. Additionally, snakes are a typical prey species for mongooses to hunt. Rikki-tikki is a threat to the snakes because of a basic food chain hierarchy.  
Nag and Nagaina definitely feel the potential threat that Rikki-tikki represents once Rikki-tikki proves he is more than capable of dealing with a snake. First, Rikki-tikki proves his quickness by dodging Nagaina's sneaky attack from behind. Second, Rikki-proves his ability to kill when he kills Karait.  
The final way that Rikki-tikki is a threat to Nag and Nagaina is that a mongoose is an opportunistic predator. A mongoose survives by hunting many different prey, which doesn't bother the snakes. What does threaten the snakes is that a mongoose will often raid nests and eat the eggs. Nag and Nagaina have chosen this particular yard to lay their eggs and raise their young. Rikki-tikki not only threatens the adults, but he also threatens the lives of the unborn snakes.