PURPORT

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 yajna and accepted as part and parcel of the Supreme Personality of Godhead? Therefore the priests requested King Prthu not to kill him.