sukhinaḥ kṣatriyāḥ pārtha
labhante yuddham īdṛśam (Bhagavad Gita 2.32)
O Pārtha, happy are the kṣatriyas to whom such ﬁghting opportunities come unsought, opening for them the doors of the heavenly planets.
atha cet tvam imaṁ dharmyaṁ
saṅgrāmaṁ na kariṣyasi
tataḥ sva-dharmaṁ kīrtiṁ ca
hitvā pāpam avāpsyasi (Bhagavad Gita 2.33)
If, however, you do not perform your religious duty of ﬁghting, then you will certainly incur sins for neglecting your duties and thus lose your reputation as a ﬁghter.
Kesava Kasmiri’s Commentary
Now Lord Krishna responds to Arjunas previous distress of not wanting to slay his enemies such as Bhishma and Drona but instead to allow his enemies to slay him. The use of the word atha is to emphasise another point of view that if Arjuna declines to fight this righteous war and chooses to disregard the acquisition of happiness in either this world or the heavenly worlds as enjoined in the Vedic scriptures which state that the royal orders should conquer over his enemies and rule over the earth. Then by refusing to accept his responsibility and avoiding the battle Arjuna would be abandoning his duty which brings rewards and boundless glory and thus losing his reputation both worldly and divine which results from the victory of a great warrior Arjuna would in fact incur great sin.