"Once upon a time, however, the King of heaven, Indra, understood that Visvarupa was secretly cheating the demigods by offering oblations on behalf of the demons. He became extremely afraid of being defeated by the demons, and in great anger at Visvarupa he cut Visvarupa's three heads from his shoulders."
"Because of affection for the demons, Visvarupa secretly supplied them the remnants of yajna. When Indra learned about this, he beheaded Visvarupa, but he later regretted killing Visvarupa because Visvarupa was a brahmana. Although competent to neutralize the sinful reactions for killing a brahmana, Indra did not do so. Instead he accepted the reactions. Later, he distributed these reactions among the land, water, trees and women in general. Since the land accepted one fourth of the sinful reactions, a portion of the land turned into desert. The trees were also given one fourth of the sinful reactions, and therefore they drip sap, which is prohibited for drinking. Because women accepted one fourth of the sinful reactions, they are untouchable during their menstrual period. Since water was also infested with sinful reactions, when bubbles appear in water it cannot be used for any purpose.
After Visvarupa was killed, his father, Tvasta, performed a sacrifice to kill King Indra. Unfortunately, if mantras are chanted irregularly, they yield an opposite result. This happened when Tvasta performed this yajna. While performing the sacrifice to kill Indra. Tvasta chanted a mantra to increase Indra's enemies, but because he chanted the mantra wrong, the sacrifice produced an asura named Vrtrasura, of whom Indra was the enemy. When Vrtrasura was generated from the sacrifice, his fierce features made the whole world afraid, and his personal effulgence diminished even the power of the demigods. Finding no other means of protection, the demigods began to worship the Supreme Personality of Godhead, the enjoyer of all the results of sacrifice, who is supreme throughout the entire universe. The demigods all worshiped Him because ultimately no one but Him can protect a living entity from fear and danger. Seeking shelter of a demigod instead of worshiping the Supreme Personality of Godhead is compared to trying to cross the ocean by grasping the tail of a dog. A dog can swim, but that does not mean that one can cross the ocean by grasping a dog's tail.
Being pleased with the demigods, the Supreme Personality of Godhead advised them to approach Dadhici to beg him for the bones of his own body. Dadhici would comply with the request of the demigods, and with the help of his bones Vrtrasura could be killed."
Srimad-Bhagavatam 6:9 Summary
"Vrtrasura was very powerful in physical strength and influence. He placed his lower jaw on the ground and his upper jaw in the sky. His mouth became very deep, like the sky itself, and his tongue resembled a large serpent. With his fearful, deathlike teeth, he seemed to be trying to devour the entire universe. Thus assuming a gigantic body, the great demon Vrtrasura shook even the mountains and began crushing the surface of the earth with his legs, as if he were the Himalayas walking about. He came before Indra and swallowed him and Airavata, his carrier, just as a big python might swallow an elephant."
"With his thunderbolt, King Indra, who was also extremely powerful, pierced through Vrtrasura's abdomen and came out. Indra, the killer of the demon Bala, then immediately cut off Vrtrasura's head, which was as high as the peak of a mountain."
"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."
Srimad-Bhagavatam 6:13:3 Purport
"After killing Vrtrasura, Indra could not surpass the brahma-hatya, the sinful reactions for killing a brahmana. Formerly he had killed one brahmana, Visvarupa, out of circumstantial anger, but this time, following the advice of the sages, he killed another brahmana purposely. Therefore the sinful reaction was greater than before. Indra could not be relieved from the reaction simply by performing sacrifices for atonement. He had to undergo a severe series of sinful reactions, and when he was freed by such suffering, the brahmanas allowed him to perform the horse sacrifice. The planned execution of sinful deeds on the strength of chanting the holy name of the Lord or undergoing prayascitta, atonement, cannot give relief to anyone, even to Indra or Nahusa. Nahusa was officiating for Indra while Indra, absent from heaven, was going here and there to gain release from his sinful reactions."
Srimad-Bhagavatam 6:13:10 Purport
Bhaktivedanta Book Trust. Excerpted from text and purport of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.