Bhagavad Gita · Chapter 1: Arjuna's Despair

Bhagavad Gita 1.36

nihatya dhārtarāṣṭrānnaḥ kā prītiḥ syājjanārdana | pāpamevāśrayedasmānhatvaitānātatāyinaḥ ||

What joy could we have, Krishna, in killing the sons of Dhritarashtra? Only sin would cling to us for slaying these aggressors.

Modern context

Arjuna himself calls them aggressors and in the same breath says that punishing them would be a sin. He knows the wrong that was done and still cannot bear to be the one who answers it. Knowing what is right and being able to act on it are two different things, and the Gita is addressed to the gap between them.