SB 6.12.16
pasya mam nirjitam satru
vrknayudha-bhujam mrdhe
ghatamanam yatha-sakti
tava prana-jihirsaya
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O my enemy, just look at me. I have already been defeated, for my weapon and arm have been cut to pieces. You have already overwhelmed me, but nonetheless, with a desire to kill you, I am trying my best to fight. I am not at all morose, even under such adverse conditions. Therefore you should give up your moroseness and continue fighting.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.