Even choosing to do nothing also has consequences.
Arjuna believed that he could avoid sin by not fighting. However, Krishna shows that there are consequences to deviating from one's righteous duty. Inaction is not always a harmless or neutral choice.
Arjuna has a svadharma, which is the duty to uphold righteousness and justice. If he abandons this due to fear or hesitation, it creates responsibility for not protecting what should be protected. This passage draws attention not only to the sin of action but also to the responsibility that arises from inaction.
Japanese Translation from the Geeta Press Edition - Chapter 2, Section 33: "If You Reject the Righteous Battle, You Will Incur Sin."
In this passage, after explaining the benefits of participating in such a righteous battle, the Lord will then describe the harm that would result if Arjuna were to refuse to participate.
Sanskrit original text
अथ चेत्त्वमिमं धर्म्यं संग्रामं न करिष्यसि ।
ततः स्वधर्मं कीर्तिं च हित्वा पापमवाप्स्यसि ॥ ३३ ॥Romanization
atha cet tvam imaṃ dharmyaṃ saṅgrāmaṃ na kariṣyasi |
tataḥ svadharmaṃ kīrtiṃ ca hitvā pāpam avāpsyasi || 33 ||Now, if you refuse to fight this righteous battle, then at that time you will abandon your duty, lose your honor, and incur sin. (33)
In this section, the word "Atha" is used to introduce a different perspective. It indicates that the validity of Arjuna's participation in the battle can be demonstrated from another point of view as well.
In the previous two sections, the Lord has made it abundantly clear to Arjuna that the battle he is facing is a righteous one, and participating in it is his duty.
Now, the Lord states that even so, if he withdraws from the battle, he will incur the sin of falling away from the duty assigned to a Kshatriya, which is Svadharma.
Furthermore, the victories he achieved against demons such as Nivartakavacha, and the worldwide fame he gained through his conflicts with Bhagavan Shiva, would be completely destroyed and lost.
In addition, failing to fulfill one's duties makes him a sinful person.
Therefore, the fear of sin that he cited as a reason for withdrawing from the battle, and his anxiety and terror, were completely unjustified and untimely.