The explanation is simple. Indra, it seems to me, is a god of just the same sort as Zeus, whose nature and history I have already explained according to my lights. In the far-away past Indra was simply a hero: very likely he was once a chieftain on earth. The story of his great deeds so fascinated the imagination of men that they worshipped his memory and at last raised him to the rank of a chief god. Now they had previously worshipped two very high gods; one of these was Dyaush-pita, the Sky-father, of whom I have spoken before, and another was Tvashta, the All-creator. So some of them, as the Rig-veda proves, declared that Dyaus was the father of Indra, and others appear to have given this honour to Tvashta, while others regarded Tvashta as Indra's grandfather; and some even said that in order to obtain the soma to inspire him to divine deeds Indra killed his father, which of course is just an imaginative way of saying that Indra was made into a god and worshipped in place of the elder god.
The puzzle now is solved. Indra has remained down to the time of the Rig-veda true to his early nature, an epic hero and typical warrior; but he has also borrowed from the old Sky-father the chief attributes of a sky-spirit, especially the giving of rain and the making of light, which the priests of the Rig-veda riddlingly describe as setting free the waters and the cows. He bears the thunderbolt, as does also Zeus; like Zeus, he has got it from the Sky-father, who had likewise a thunderbolt, according to some Rigvedic poets, though others say it was forged for him by Tvashta, his other father. I even venture to think that there is a kernel of heroic legend in the story of the slaying of Vritra; that at bottom it is a tale relating how Indra with a band of brave fellows stormed a mountain hold surrounded by water in which dwelt a wicked chieftain who had carried away the cattle of his people, and that when Indra had risen to the rank of a great god of the sky men added to this plain tale much mythical decoration appropriate to his new quality, turning the comrades of Indra into the storm-gods and interpreting the waters and cows to mean rain and daylight. Since most of us are agreed that stories such as that of Indra defeating Sambara for the benefit of Divodasa refer to real events, it seems unnatural to suppose that the Vritra-legend is a purely imaginary myth. We can thus explain why the ideas of Indra setting free the rain and the light fit in so awkwardly with the heroic element in the legend: for they are merely secondary attributes, borrowed from the myths of other gods and mechanically attached to Indra on his elevation in the pantheon. But we can explain much more. There is a regular cycle of hero-saga connected with Indra which is visible or half-visible at the back of some of the Vedic hymns and of the priestly literature which is destined to follow them.