Why do you suppose Rikki-tikki does not want Darzee to sing of Nag's defeat?

Rikki-tikki does not want Darzee to sing of Nag's defeat because it could attract the attention of Nagaina, Nag's mate, who may seek revenge on Rikki-tikki for killing Nag. Rikki-tikki knows that remaining discreet and not drawing attention to his actions is key to his safety.