Killing a brahmin is the gravest offence the ritual literature knows, and Indra has done it. The consequences are worked out at length in the later texts, and they are worked out as a problem of accounting rather than of conscience: the guilt is a substance, it exists, and it has to go somewhere. In the standard treatment it is divided and distributed, to the earth, to the trees, to the waters and to women, each of which receives a share along with a compensating boon. The god is cleared. The cost is spread across the world.
The second consequence is narrative. Tvashtri, who made the thunderbolt Indra kills with, has now lost a son to it, and the tradition has him produce Vritra afterwards specifically to avenge the killing. That turns the central deed of Vedic myth, the slaying of the serpent and the release of the waters, into the second half of a revenge sequence rather than a self contained triumph. The artisan supplies the weapon, then supplies the enemy.
This is the part of the material that the epic and Puranic literature found most useful, because it gives Indra a moral history. A king of the gods who has incurred brahminicide, been purified by a distribution of guilt, and then killed the avenger sent against him, is a very different figure from the straightforward champion of the earliest hymns. The atlas keeps the sequence together for that reason. Vishvarupa is a minor figure by any count of verses, and removing him would leave the two most important things about Indra unexplained.