Originally posted by: Chandraketu
I am red faced 😳😳😳- the Brahmin Indra killed wasn't Vritra (who was an asura), but Trisiras, the 3 headed son of Tvastha. Story goes that there was a fallout between Indra & Brihaspati, & the latter left Devalok. As a result, the asuras got the upper hand in their battles against the devas, and Indra needed a suitable guru. He approached Trisiras, the son of Tvastha, who also had asura parentage via his mother. So while Trisiras initially did the yagnas properly, the success of the devas actually saddened him, since the asuras were his mother's kith & kin. So he started doing the yagnas to benefit both devas & asuras, and soon, the devasura battles started becoming stalemates. Indra was perplexed at this situation, and then did an yagna which was to redeem that situation. In that yagna, Trisiras poured a spoon of ghee in for the devas, while chanting the mantras, but next, emptied the entire pot of ghee in for the asuras. Indra was furious and instantly beheaded Trisiras. His 3 heads became 3 birds (forget which type) that flew away. Rishi Tvastha divined what Indra had done, and created Vritra for the sake of killing him. Indra in turn got Rishi Dadhichi to sacrifice his life so that Indra could make a mace out of his bones: reason he needed this was that Dadhichi was the only rishi whose tapas matched that of Tvastha.
Bottom line - after Vritra was killed and it was all over, Indra did the ashwamedha to atone for the killing of Trisiras.