I suppose one could say that, of the cobras that Rikki-tikki-tavi fights, Nagaina is the more sympathetic. Once he's killed the fearsome king cobra Nag, Rikki-tikki-tavi descends on Nagaina's eggs and initially destroys all but one of them. Nagaina begs the mongoose to spare her last remaining egg. She promises Rikki-tikki that if he lets her and her baby go, she'll never come back. But Rikki-tikki doesn't believe her. Nagaina is, after all, a cobra. And so, as long as Rikki-tikki is around, the cobras will always regard him as a threat. So Nagaina's promise rings somewhat hollow. Realizing that the mongoose won't play ball, she slithers off carrying her egg. But Rikki-tikki is in hot pursuit and follows Nagaina into her den, where he kills her and destroys the egg.
We can understand why Rikki-tikki did what he did; he was only trying to protect the human family which had adopted him. But Nagaina was also trying to protect her family, and her desperate pleas for Rikki-tikki to spare her last remaining egg make her much more sympathetic than the hissing, slithering bringer of death that is Nag, who himself killed one of the tailor-bird's babies.