Suras-asuras are yin-yang of Hinduism. Both are good and bad, depends on point of view. This is similar to making one hero and opponent villain.
The ancient sage Kashyap had two wives, Diti and Aditi. The Asuras were born of the first and the Suras of the other. They were, thus, initially related and, like many relatives we know, had many clashes and guarded secrets. Their gurus (Brihaspati for the Suras and Shukracharya for the Asuras) remained great friends. Both asuras and devas (suras) fight with one another for land or wealth.