But if you do not perform this duty-conforming battle, then having abandoned your duty and your fame, you will incur demerit

The consequence of omission: na kariṣyasi (you will not do) implies apapuṇyam (bad fortune, meritless action). The root apuṇya (without-merit) describes karmic fall.

The loss is double: sva-dharma (own duty) and kīrti (fame, reputation). The warrior without battle loses social and spiritual identity.

Atha cet (but if) introduces the counterfactual condition. Arjuna has a choice, but with defined negative consequences.