Arjun Vishaad Yog-Bhagavad Gita Chapter 1 [Observing the Armies on the Battlefield of Kurukshetra]
Arjun Said > Shaloka: 40
With the destruction of dynasty, the eternal family tradition is vanquished, and thus the rest of the family becomes involved in irreligious practice.
In the system of the varnasrama 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.