Bhagavad Gita · Sankhya YogaChapter 2, Shloka 21 (2.21)

Two actions — killing and causing to kill. Arjuna's grief was not only about killing with his own hands; he was one of the commanders — on his orders others too would kill. So Krishna mentions both: hanti (killing oneself) and ghātayati (having another kill).

The form of the question — "katham". Shankaracharya says that "how" here is not a real question but an objection (a denial): "How will he kill? — in no way at all."

A major difference between the traditions — what does this shloka conclude?

• Advaita (Shankara): According to Shankaracharya, the main purpose of this section is — viduṣaḥ sarva-karma-pratiṣedhaḥ — the negation of all action for the knower of the self. "Killing" is only an example. Since the self is actionless, one who knows it is not the doer of any action; so the wise person is entitled to sannyasa (renunciation of action). For Arjuna (who is not yet a knower), the teaching of karma-yoga comes later (from 2.39).
• Vishishtadvaita (Ramanuja): Ramanuja says that one who knows the eternity of the self does not grieve, saying "I am killing them" — this grief comes from ignorance of the true nature of the self. Ramanuja adds that in war there is separation from the body, but according to scripture those who give up the body in a righteous war attain a better body (on 2.22) — so there is no cause for grief. That is, for Ramanuja the conclusion is not giving up action, but giving up grief and doing one's duty in the right spirit.

This difference is one of the central questions in interpreting the Gita — "does action remain after knowledge, or not?"