Purport

Bhagvan Shri Krishna further explains Arjuna about the impact of not performing duty. As a Kshatriya fighting for the welfare of Dharma and humanity is his natural duty. If however he ignores his duty at such a critical time which might have lot of consequence, leaving his duty makes him liable for sin.
Also since he is a warrior and all his reputation and fame is due to his skills of fighting, If he does not fight would lose all the reputation in society.

This verses though is specifically for the Dutry of Arjuna as a warrior, but same applies to all kind of duties. Every human must perform his duty. Not performing duties is one of the reasons of downfall of society.