In the RigVeda the Asuras are not yet demons. At that time the Asuras and the Devas were both still groups of gods. In fact, many of the figures of the RigVeda are both asuras and daevas. This is because, at that time, the two words were not yet names of groups. They were just names of characteristics. "Asura" means "life", and so the gods who were called asuras were thought to give life. And the daevas were shining. This is the original meaning of the names. In later books they became different.
In the RigVeda many of the devas are called asuras, and many of the asuras are called devas. Sometimes they are also called both at the same time: Like "daevic asuras" or "asuric devas". The "-ic" makes those words a part of the other word. There are some beings who are both ahuric daevas and daevic ahuras. This shows that there was no big difference between 'ahura' and 'daeva' in the RigVeda.
The forefathers of the people who wrote the Vedas spoke Sanskrit. Sanskrit is the source of many Indian languages. The stories in the Rig Veda are seen in the books of various North Indian peoples, and also in the books of some Iranian peoples like the Zoroastrians.
One of the books of the Iranian peoples is called the Avesta. It is not really one book, but a collection of many books, but together they are called the Avesta. The Avesta is the holy book of Zoroastrianism. The language of the Avesta is very similar to the language of the RigVeda. The Avesta also has asuras and devas. But in Iran they are written as "ahura" and "daeva".
The old part of the Avesta is almost as old as the RigVeda. In this old part of the Avesta the ahuras and the daevas are almost like they are in the RigVeda. In both books they are groups of supernatural beings. But they are very different in character.
Some time after the RigVeda was written, stories were told about the asuras and devas that made them not friends any more. A similar thing also happened in the Avesta. Only that the two groups had been switched: The Asuras became demons in regions where North Indian languages were spoken, and the Daevas became demons in regions where Iranian languages were spoken.
It is not known for sure why this happened. It was a very, very long time ago.
One possible reason why the Asuras became demons in India is this: Some time after the RigVeda was written Indians started to think that asura means a-sura, which means "not a hero". So, maybe that way the asuras got a bad name.
One possible reason why the Daevas became demons in Iran is this: At about the same time as the Indians were writing the last part of the RigVeda, a very thoughtful man named Zoroaster appeared in Iran. He thought and thought a lot about why there was goodness and badness in the world. Finally, he told everyone what he had been thinking about. And one of those things was that Daevas were not to be trusted because they could not tell the difference between truth and lies. After some time Zoroaster became very popular for his ideas. And so, with time, the Daevas became demons in Iran.
In both cases, the one side did not know what the other side was thinking. So in India only the Asuras became demons. And in Iran only the Daevas became demons.
As fake as a ladyboys orgasm.
