This self that dwells in the body of every being, O descendant of Bharata, can never be killed. Therefore you should not grieve for any living being.
Explanation
Krishna concludes his first argument to Arjuna, who is overwhelmed with grief about fighting his own relatives. The embodied one (dehī), the self (ātman) that dwells in every body, cannot be slain, so Krishna says there is no reason to mourn for any being. Here the verse speaks of the body's occupant being beyond harm, not of denying that death and loss are painful.