PURPORT

Vrtrasura was so great and powerful that in effect he was acting as the spiritual master of Indra. Although Vrtrasura was on the verge of defeat, he was not at all affected. He knew that he was going to be defeated by Indra, and he voluntarily accepted that, but since he was supposed to be Indra's enemy, he tried his best to kill Indra. Thus he performed his duty. One should perform his duty under all circumstances, even though one may know what the result will be.