SB 6.13.10
sri-suka uvaca
evam sañcodito viprair
marutvan ahanad ripum
brahma-hatya hate tasminn
asasada vrsakapim
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sri Sukadeva Gosvami said: Encouraged by the words of the sages, Indra killed Vrtrasura, and when he was killed the sinful reaction for killing a brahmana [brahma-hatya] certainly took shelter of Indra.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.