|Chapter 2: Contents of the Gita Summarized|
vasamsi jirnani yatha vihaya
navani grihnati naro 'parani
tatha sarirani vihaya jirnany
anyani samyati navani dehi
As a person puts on new garments, giving up old ones, the soul similarly accepts new material bodies, giving up the old and useless ones.
Change of body is inevitable. Over the course of time, the souls in the body of Bhishma and Drona will be forced to give up there old and useless bodies and accept new ones. Therefore Arjuna should not let this consideration prevent him from discharging his prescribed duty. The execution of prescribed duty is so glorious, that even the so called victim benefits. If a judge orders the execution of a murderer, then not only does the judge get the pious benefits of performing his prescribed duties, the execution relieves the criminal from further suffering the reactions of the crime.
Srila Prabhupada comments, "Arjuna is advised herewith by the Lord not to lament for the bodily change of his old grandfather and his teacher. He should rather be happy to kill their bodies in the righteous fight so that they may be cleansed at once of all reactions from various bodily activities. One who lays down his life on the sacrificial altar, or in the proper battlefield, is at once cleansed of bodily reactions and promoted to a higher status of life. So there was no cause for Arjuna's lamentation."
Please read the Sanskrit Verse and the Prabhupada's Purport.