Why does Nag hide in the bathroom in "Rikki-Tikki-Tavi"?

Nag hides in the bathroom in order to ambush and kill the man of the house.  
By this point in the story, Rikki-tikki has proven that he is a threat to Nag, Nagaina, and their eggs. He killed Karait and successfully dodged an attack from behind. Both instances show Rikki-tikki is a threatening, quick predator. Nag and Nagaina need a way to get rid of Rikki-tikki. Their plan involves killing the people in the house. They both believe that the threat that Rikki-tikki is to them will go away once the people are killed. Nagaina believes Rikki-tikki will be easier to hunt with no people around.

"Go in quietly, and remember that the big man who killed Karait is the first one to bite. Then come out and tell me, and we will hunt for Rikki-tikki together."

Nag believes that Rikki-tikki will simply leave of his own volition if the house is devoid of people.  

"I will kill the big man and his wife, and the child if I can, and come away quietly. Then the bungalow will be empty, and Rikki-tikki will go."

Either way, Nag and Nagaina agree that the man needs to be killed first. Nag's plan is to ambush the man in the bathroom because the man is not likely to be carrying a defensive weapon there.  

Now, when Karait was killed, the big man had a stick. He may have that stick still, but when he comes in to bathe in the morning he will not have a stick. I shall wait here 'till he comes.