The Asuras are generally considered divine beings, who are primarily known for doing evil, but not always. It might be better to say that the Asuras are powerful beings who often are opposed to the gods. By the end of the Vedic period, however, the asuras had attained their more demonic role.
Certainly some of the Asuras were quite evil, but some were pious. They would, at times, even be more powerful than the gods, forcing them to flee in the face of their power until some way of dealing with them could be found.
A per Puranic traditions, Asuras and Devas were stepsons of sage Kashyap. It is also said that in some Suktas (sections) of Rigveda, perhaps older ones, Asuras also have been praised.
However, in most of the Suktas ( perhaps later ones) Asuras have been projected as enemies. However, in Puranas Asuras have been described no less virtuous than Devas. However, they were too proud and due to that they suffered at the hands of Devas.
Further and curiously enough, Iranians, who are said to be clansmen of Indo Aryans, worshiped Asur Mazda and depicted Devas as villains.
It is not clear how this transformation came about. Zarathustra is said to have propagated the worship of Asur Mazda. However, his idea was to propagate the worship of one God, instead of worship of many Gods, as was the practice among Proto Indo-Iranians. Perhaps this was the practice among Indo-Europeans also.
Mittani kings of ancient Iraq also worshipped Devas. Worship of Devas by Mittani kings proves that the concept of Devas is older than Asuras.
As per Indian tradition, Asuras are also called ‘Danavas’ as their mother’s name was ‘Danu’. Both the words that is ‘Asura’ and ‘Danava’ are synonyms. It is interesting to note that we have a very important river ‘Danube’ in Europe. This may be a combination of words ‘Danu’ and ‘Aab’. We also have a European tribe named ‘Danes’.
Further, there is one more important branch of Kashyap’s family; that is Daityas, sons of ‘Dity’ the third wife of Kashyap. We also have a tribe named ‘Dutch’ in Europe. This word could either be transformation of Daitya or Dadhichi; an ancient sage of India. The word Europe also seems to has been derived from the word ‘Aila’( descendents of ‘Ila’).
In conclusion I can say that Indian belief needs to be studied in more depth than presently being done.