Darzee is a tailorbird who lives in the garden with his wife. Like most of the animals who live there, he is absolutely petrified of Nag and Nagaina, not least because Nag ate one of Darzee and his wife's babies when it fell out of the nest.
One can only imagine, then, how elated Darzee is when the fearless mongoose Rikki-Tikki-Tavi kills Nag. Indeed, he's so overcome with joy that he sings a song of triumph. Rikki's none too impressed, however. He knows how foolish it is to get all excited when Nagaina's still on the loose. So long as she's around, with her eggs ready to hatch, none of the animals in the garden will be safe, including Darzee and his wife.
So Rikki sets about taking care of unfinished business, namely getting rid of Nagaina. But first, he plans to creep into her nest and destroy her eggs. To this end, he asks Darzee for assistance. He wants the tailorbird to distract Nagaina by pretending to be injured, thus giving Rikki the opportunity to sneak into the cobra's nest and destroy all her eggs.
But Darzee's decidedly uncomfortable with Rikki's plan. He doesn't understand the difference between birds' eggs and cobras' eggs and so feels it rather unfair to destroy either of them. Even though the cobras that will emerge from Nagaina's eggs will eventually pose a considerable danger to Darzee and his family, he still feels uneasy about destroying them.