Text 391 Observing the Armies on the Battlefield of Kuruksetra 63
PURPORT
A  kṣatriya  is not supposed to refuse to battle or gamble when he is so invited by some rival party. Under such obligation, Arjuna could not refuse to fight because he was 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.
TEEXT 39
कुलक्षये प्रणश्यन्ति कुलधर्माः सनातनाः । धर्मे नष्टे कुलं कृत्स्नमधर्मोऽभिभवत्युत ॥३९॥
kula-kṣaye praṇaśyanti  
 kula-dharmāḥ sanātanāḥ  
 dharme naṣṭe kulaṁ kṛtsnam  
 adharmo’bhibhavaty uta
kula-kṣaye —in destroying the family;  praṇaśyanti —becomes vanquished;  kula-dharmāḥ —the family traditions;  sanātanāḥ —eternal;  dharme —in religion;  naṣṭe —being destroyed;  kulam —family;  kṛtsnam —wholesale;  adharmaḥ —irreligious;  abhibhavati —transforms;  uta —it is said.
TRANSLATION
With the destruction of dynasty, the eternal family tradition is vanquished, and thus the rest of the family becomes involved in irreligious practice.
PURPORT
In the system of the  varṇāśrama  institution there are many principles of religious traditions to help members of the family grow properly and attain spiritual values. The elder members are responsible for such purifying processes in the family, beginning from birth to death. But on the death of the elder members, such family traditions of purification may stop, and the remaining younger family members may develop irreligious habits and thereby lose their chance for spiritual salvation. Therefore, for no purpose should the elder members of the family be slain.