srotum icchami bho mune
yenasan sukhino deva
harer duhkham kuto ’bhavat
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Maharaja Pariksit inquired from Sukadeva Gosvami: O great sage, what was the reason for Indra’s unhappiness? I wish to hear about this. When he killed Vrtrasura, all the demigods were extremely happy. Why, then, was Indra himself unhappy?
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
This, of course, is a very intelligent question. 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, Indra was unhappy. Why? It may be suggested that Indra was unhappy because he knew that he had killed a great devotee and brahmana. Vrtrasura outwardly appeared to be a demon, but inwardly he was a great devotee and therefore a great brahmana.
Herein it is clearly indicated that a person who is not at all demoniac, such as Prahlada Maharaja and Bali Maharaja, may outwardly be a demon or be born in a family of demons. Therefore in terms of real culture one should not be considered a demigod or demon simply according to birth. In his dealings while fighting with Indra, Vrtrasura proved himself a great devotee of the Supreme Personality of Godhead. Furthermore, as soon as he finished fighting with Indra and was apparently killed, Vrtrasura was transferred to Vaikunthaloka to become an associate of Sankarsana. Indra knew this, and therefore he was morose at having killed such a demon, who was actually a Vaisnava or brahmana.
A Vaisnava is already a brahmana, although a brahmana may not be a Vaisnava. The Padma Purana says:
avaisnavo gurur na syad
vaisnavah sva-paco guruh
One may be a brahmana in terms of his culture and family and may be expert in Vedic knowledge (mantra-tantra-visaradah), but if he is not a Vaisnava, he cannot be a guru. This means that an expert brahmana may not be a Vaisnava, but a Vaisnava is already a brahmana. 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 brahmana.