What does Nagaina do to make matters worse for Nag and herself?

When Rikki- tikki first meets Nag and Nagaina, he bites Nagaina, but releases her tail too early. She survives, although she is injured and very angry. Nagaina then plots to kill the human family.  She feels that if the humans are gone, then the mongoose will leave also. That will leave the garden safe for her, Nag, and their children.  Nag is not too sure that anything will be gained by killing the humans, but...

When Rikki- tikki first meets Nag and Nagaina, he bites Nagaina, but releases her tail too early. She survives, although she is injured and very angry. Nagaina then plots to kill the human family.  She feels that if the humans are gone, then the mongoose will leave also. That will leave the garden safe for her, Nag, and their children.  Nag is not too sure that anything will be gained by killing the humans, but Nagaina convinces him.  Her main concern is for her eggs.  They are on the verge of hatching, and,

“……our children will need room and quiet.”  (pg 5)

Nag had not thought of that, so he enters the house with the goal of killing the humans.

“I will kill the big man and his wife, and the child if I can, and come away quietly. The bungalow will be empty, and Rikki-tikki will go” (pg 5)

However, Rikki-tikki had been warned by Chuchundra, the muskrat.  Chuchundra alerted Rikki-tikki to the sound of Nag  and Nagaina’s scales scrapping against the floor. Rikki-tikki heard  their plans. When Nag enters the bungalow, he hides in the water-jar in the bathroom.  Rikki-tikki waits until Nag falls asleep and then jumps on his back and holds on for dear life.  The noise brings the man of the house, who shoots Nag with a shotgun.  Rikki-tikki didn’t think that was necessary because he had already killed Nag.

Nag’s body is tossed onto the rubbish heap, and Nagaina goes there to mourn the death of her husband. When she does so, she leaves her eggs unprotected.  Rikki-tikki sends Darzee’s wife to pretend she has a broken wing and keep Nagaina busy while he goes and bites the ends off of all but one of her eggs, killing all the cobras inside.  Rikki-tikki saves one egg, and entices Nagaina away from Teddy, the human child, with it.  Nagaina rescues the egg and dives into her cobra hole with Rikki-tikki biting her tail.  She drags him into the hole.  Rikki-tikki emerges, and Nagaina and her egg are dead. So, Nagaina’s decision to attack the family in hopes of getting rid of Rikki-tikki resulted in Nag’s death, her death, and that of her children.