Darzee and Nagaina are similar in the fact that they both recognize a threat to their family, and they are prepared to kill innocents to keep their family safe.
Nagaina is afraid of Rikk-tikki. She knows he is a natural predator to snakes. She knows her eggs are in danger as long as Rikki-tikki is around, so she comes up with a plan to get rid of Rikki-tikki. She and Nag will kill the humans. She knows that once the humans are gone from the house, Rikki-tikki is not likely to stay there.
"But are you sure that there is anything to be gained by killing the people?" said Nag.
"Everything. When there were no people in the bungalow, did we have any mongoose in the garden? So long as the bungalow is empty, we are king and queen of the garden."
Darzee's wife has the exact same attitude. She knows cobras are a threat to her and her eggs. She also knows a bunch of baby cobras is even more dangerous. That's why she has no problem with distracting Nagaina. While Darzee's wife fakes having a broken wing, Rikki-tikki is able to sneak over to Nagaina's nest and destroy most of the eggs. Darzee's wife has no problem with helping kill a bunch of baby snakes because it will protect her family.