Sankhaya Yog-Bhagavad Gita Chapter 2 [Contents of the Gita Summarized]
Krishna Said > Shaloka: 33
If, however, you do not fight this religious war, then you will certainly incur sins for neglecting your duties and thus lose your reputation as a fighter.
Arjuna was a famous fighter, and he attained fame by fighting many great demigods, including even Lord Siva. After fighting and defeating Lord Siva in the dress of a hunter, Arjuna pleased the lord and received as a reward a weapon called pasupata-astra. Everyone knew that he was a great warrior. Even Dronacarya gave him benedictions and awarded him the special weapon by which he could kill even his teacher. So he was credited with so many military certificates from many authorities, including his adopted father Indra, the heavenly king. But if he abandoned the battle, he would not only neglect his specific duty as a ksatriya, but he would lose all his fame and good name and thus prepare his royal road to hell. In other words, he would go to hell, not by fighting, but by withdrawing from battle.