Rikki-tikki-tavi is a mongoose, so killing and eating snakes is pretty much what he does. The case of Karait is different, though (although he does in fact kill the young brown snake). Just as he's about to be patted by Teddy, Rikki-tikki is suddenly confronted by a little voice seething, "Be careful. I am death!" It's Karait. The mongoose doesn't hesitate; he swings into action, protecting Teddy from Karait. In a matter of seconds, the fight between Rikki-tikki and Karait is over; the snake lies dead. Naturally, Rikki-tikki wants to eat the snake, but then he remembers that a full meal makes for a slow mongoose. In other words, if he's too full from eating Karait, he won't be quick enough to carry out his duties in defending Teddy and his family from other snakes.