Why were Darzee and his wife crying?

Shortly after Rikki-tikki is adopted into his new home, he explores the bungalow's garden and hears sorrowful voices coming from the thorn bush. When Rikki-tikki explores the location of the cries, he discovers Darzee the Tailorbird and his wife in a miserable condition. Rikki-tikki proceeds to ask why they are crying, and Darzee tells Rikki-tikki that one of their babies fell out of their nest yesterday and Nag, the malevolent male king cobra, ate their child. Immediately after Rikki-tikki asks Darzee and his wife who Nag is, Nag strikes at Rikki-tikki, who narrowly dodges his bite by acrobatically jumping back in the air. Nag then presents himself by spreading his hood and showing off his imposing black body. Nag responds to Rikki-tikki's question by saying,

I am Nag. The great God Brahm put his mark upon all our people, when the first cobra spread his hood to keep the sun off Brahm as he slept. Look, and be afraid! (Kipling, 2)

Nag is one of the two wicked cobras that inhabit the bungalow's garden and attempt to harm Rikki-tikki's adoptive human family. As the story continues, Rikki-tikki protects his human family by fighting and killing both Nag and Nagaina.

Rikki-Tikki-Tavi has recently been taken in by his new adoptive family. The young mongoose immediately takes to his surroundings and starts to explore the family's large, half-cultivated garden. This is clearly a good hunting-ground, and Rikki licks his lips at the prospect.
As he continues to explore his new domain, Rikki hears some sorrowful voices emanating from a thorn-bush. It's Darzee the tailor-bird and his wife. They're very sad because one of their babies fell from their nest the previous day and was gobbled up by Nag, the big black cobra. Rikki-Tikki is keen to know more about this deadly creature. But he doesn't have long to wait before he finds out from the horse's—or rather, snake's—mouth, as Nag himself suddenly appears, spreading out his hood to intimidate the mongoose, and staring right at him with those wicked eyes of his. If Rikki didn't know who Nag was he certainly does now. And he's left in no doubt that the big black cobra's trouble with a capital T.