Bhagavad Gita 1.36 Chapter 1, Verse 36

निहत्य धार्तराष्ट्रान्नः का प्रीतिः स्याज्जनार्दन ।
पापमेवाश्रयेदस्मान्हत्वैतानाततायिनः ॥१.३६॥

nihatya dhārtarāṣṭrānnaḥ kā prītiḥ syājjanārdana
pāpamevāśrayedasmānhatvaitānātatāyinaḥ

Meaning

O Janardana, what joy could we have in killing the sons of Dhritarashtra? Even though they are aggressors, only sin would come to us by slaying them.

Newly written by Bhajan Dhun — scholarly review in progress.

Reference: Bhagavad Gita 1.36 (Chapter 1, Arjuna's Despair (Arjuna Vishada Yoga), Verse 36)