vicakṣya duṣprekṣyam asahya-raṁhasam
nivārayām āsur aho mahā-mate
na yujyate ’trānya-vadhaḥ pracoditāt
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 Viṣṇu. 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 Pṛthu not to kill him.