Bhagavad Gita Chapter 1, Verse 39

तस्मात्न अर्हा वयं हन्तुं धार्तराष्ट्रान्स्वबान्धवान् | स्वजनं हि कथं हत्वा सुखिनः स्याम माधव ||

tasmān nārhā vayaṁ hantuṁ dhārtarāṣṭrān sva-bāndhavān sva-janaṁ hi kathaṁ hatvā sukhinaḥ syāma mādhava

Therefore we ought not kill the sons of Dhritarashtra, our own kinsmen. How, having slain our own people, can we be happy, O Madhava?

Arjuna draws his conclusion: 'Tasmāt'—therefore. Having seen what his opponents miss, he states 'nārhā vayaṁ hantum'—we ought not kill. Not 'cannot' or 'will not,' but 'ought not'—a moral obligation rooted in consequence. They're 'dhārtarāṣṭrān sva-bāndhavān'—Dhritarashtra's sons and our kinsmen. Both labels apply. His devastating question: 'kathaṁ hatvā sukhinaḥ syāma'—how, having killed them, can we be happy? Not whether it's justified, but whether it leads to happiness. You can win completely and lose what makes winning worthwhile. The teaching: sometimes what you're entitled to do would make your life unlivable.