tam rtvijah sakra-vadhabhisandhitam
vicaksya duspreksyam asahya-ramhasam
nivarayam asur aho maha-mate
na yujyate ’tranya-vadhah pracoditat
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When the priests and all the others saw Maharaja Prthu very angry and prepared to kill Indra, they requested him: O great soul, do not kill him, for only sacrificial animals can be killed in a sacrifice. Such are the directions given by sastra.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Animal killing is intended for different purposes: it tests the proper pronunciation of Vedic mantras, and an animal being put into the sacrificial fire should come out with a new life. No one should ever be killed in a sacrifice meant for the satisfaction of Lord Visnu. How then could Indra be killed when he is actually worshiped in the yajña and accepted as part and parcel of the Supreme Personality of Godhead? Therefore the priests requested King Prthu not to kill him.