You have asked a question that assumes something, perhaps rihgtly, yet that is not fully clear to scholars. A rivalry between the gods and the asuras is reported in the Aitareya Brahmana but the AB is post-Rgvedic. asura  is, at least mostly, a noble concept in the Rgveda. The seed of the Puranic idea could be traced to its origin in the RV but the idea was not widely spread. As Macdonell reports it occurs four times to mean some beings opposed to the gods. But I have not checked. However, there is no mention of the term asura in the story of the Panis (RV 10.108). In the Avesta the term means 'powerrful', 'lord'. .