In the Vaikuntha world there cannot be any fight. So because Krsna wanted to exercise His fighting spirit, He sent His devotee. He became enemy and He fought. So you have to understand Krsna in that way, as Hrsikesa. He knows that unless Arjuna becomes affected family-wise, how Bhagavad-gita will be there? Therefore although Gudakesa, Arjuna, is above darkness, still, by the will of Krsna, Hrsikesa, he played just like ordinary man, affected with his family affection. Therefore Krsna in the next verse says, "My dear Partha, now you wanted to see with whom you have to fight. Now, here is Bhisma, Drona, and many other kings. All the descendants of Kuru dynasty, your Dhrtarastra's sons. Now you see very nicely and be prepared to fight with them." So this is the explanation of Hrsikesa and Gudakesa.
Lecture on Bhagavad-gita 1.24-25, London, 20 July, 1973