Bhagavad-gita 2.31 - 2.38 Lord Krsna explains specific duty as ksatriya to fight

Lord Krishna spoke of svadharma or specific duty as per religious principles. In the bodily context this specific duty is called varnashrama-dharma which becomes the cornerstone for spiritual understanding and elevates one to a higher status of life.

Arjuna was a ksatriya, one who gives protection from harm. ksat means hurt and trayate means to give protection. A ksatriya protects citizens from all kinds of difficulties. He applies law and order and rules the world under religious principles. He is even trained in forests for killing and be ready to counter religious violence.

Only out of ignorance a ksatriya refrains from fighting and becomes non-violent in the discharge of his duty. Hence Lord Krishna condemned Arjuna’s attitude of ignorance. Arjuna would certainly incur sins for neglecting his duties.

Arjuna would also lose his reputation and prestige in society if he withdrew as a fighter. He would earn ill fame of misusing Lord’s friendship. He would be declared a coward and declared non-Aryan. Infamy is worse than death.

Arjuna would attain heavenly planet if he was killed in the religious battle. He would anyway enjoy a kingdom If he conquered the enemy.

He would also not incur sin for the sake of fighting. This is because everything performed for the sake of Krsna is transcendental consciousness and there is no material reaction. Also a person who has completely surrendered himself in activities of Krsna consciousness is no longer obliged to anyone, nor is he a debtor to anyone.

Bhagavad-gita 2.29 - 2.30 Mystery of soul revealed through Sankhya yoga

Arjuna was affected by grief at death of his relatives and fear of sinful reactions. Since the soul who dwells in the body can never be slain, there was no need to grieve for any living being. Also since the instruction to fight was coming straight from the scriptures and spiritual master there was no fear of fall down.

Lord Krsna insisted on the Vedic conclusion that the living entity is the soul and not the body. Material bodies are perishable in due course of time  (antavanta ime deha) and soul is eternal and unchangeable (nityasyoktah saririnah). 

Example - The body is like a dream in which we think of flying in the sky or sitting on a chariot as a king. When we wake up we can see that we are neither in the sky nor seated on the chariot. Similarly the material body has no factual existence in relation to the eternal soul. 

The soul is amazing and the wonders of the individual soul are not easy to understand by mundane people. However the fact is that the atomic soul is within the body of every living entity be it human, animals, trees or insects as explained by the greatest authorities of knowledge.

Some people even remain misguided about the Supersoul and individual soul and out of ignorance do not accept the distinction of magnitude between them. Each of them fail in their ultimate understanding of the self.

Under the authority of a bona fide spiritual master and the causeless mercy of the pure devotees, the truth of Sankhya yoga or the science of self realization is revealed and one's life becomes successful.