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Bg 1.37-38

yady apy ete na paśyanti
lobhopahata-cetasaḥ
kula-kṣaya-kṛtaṁ doṣaṁ
mitra-drohe ca pātakam
kathaṁ na jñeyam asmābhiḥ
pāpād asmān nivartitum
kula-kṣaya-kṛtaṁ doṣaṁ
prapaśyadbhir janārdana
Synonyms: 
yadi — if; api — even; ete — they; na — do not; paśyanti — see; lobha — by greed; upahata — overpowered; cetasaḥ — their hearts; kula-kṣaya — in killing the family; kṛtam — done; doṣam — fault; mitra-drohe — in quarreling with friends; ca — also; pātakam — sinful reactions; katham — why; na — should not; jñeyam — be known; asmābhiḥ — by us; pāpāt — from sins; asmāt — these; nivartitum — to cease; kula-kṣaya — in the destruction of a dynasty; kṛtam — done; doṣam — crime; prapaśyadbhiḥ — by those who can see; janārdana — O Kṛṣṇa.
Translation: 
O Janārdana, although these men, their hearts overtaken by greed, see no fault in killing one’s family or quarreling with friends, why should we, who can see the crime in destroying a family, engage in these acts of sin?
Purport: 

A kṣatriya is not supposed to refuse to battle or gamble when he is so invited by some rival party. Under such an obligation, Arjuna could not refuse to fight, because he had been challenged by the party of Duryodhana. In this connection, Arjuna considered that the other party might be blind to the effects of such a challenge. Arjuna, however, could see the evil consequences and could not accept the challenge. Obligation is actually binding when the effect is good, but when the effect is otherwise, then no one can be bound. Considering all these pros and cons, Arjuna decided not to fight.