Why Was Indra Unhappy After Killing Vrtrasura? by His Divine Grace A.C.Bhaktivedanta Swami Srila Prabhupada
When a demon is killed, certainly all the demigods are happy. In this case, however, when all the demigods were happy because of Vrtrasura’s having been killed, the chief of the demigods Indra, was unhappy. Why was that ? It may be suggested that Indra was unhappy because he knew that he had actually killed a great devotee and brāhmaṇa.
Outwardly Vrtrasura appeared to be a demon, but inwardly he was a great devotee and therefore a great brāhmaṇa. Herein it is clearly indicated that a person who is not at all demoniac, such as Prahlada Maharaja and Bali Maharaja, may outwardly appear to be a demon or to be born in a family of demons. Therefore in terms of Vedic culture one should not be considered a demigod or demon simply because of his birth. While fighting with Indra, Vrtrasura proved himself a great devotee of the Supreme Personality of Godhead by his dealings. Furthermore, as soon as he finished fighting with Indra and was apparently killed, Vrtrasura was transferred to Vaikuṇṭhaloka to become an associate of Sankarsana. Indra intuitively knew this, and therefore he was morose at having killed such a demon, who was covertly a Vaisnava or brāhmaṇa.
A Vaisnava is already a brāhmaṇa, although a brāhmaṇa may not be a Vaisnava. The Padma Purāṇa says:
avaiṣṇavo gurur na syād
Vaisnavaḥ śva-paco guruḥ
One may be a brāhmaṇa in terms of his culture and family and may be expert in Vedic knowledge (mantra-tantraviśāradaḥ), but if he is not a Vaisnava, he cannot be accepted as guru. This means that an expert brāhmaṇa may not be a Vaisnava, but a Vaisnava is already a brāhmaṇa. A millionaire may very easily possess hundreds and thousands of dollars, but a person with hundreds and thousands of dollars is not necessarily a millionaire. Vrtrasura was a perfect Vaisnava, and therefore he was also a brāhmaṇa.
— Śrīmad-Bhāgavatam (Bhāgavata Purāṇa) » Canto 6: Prescribed Duties for Mankind » SB 6.13: King Indra Afflicted by Sinful Reaction » Purort to the Verses: 3.