2:3 Life Force is Indestructible
Krishna told Arjuna that a moral person does what he must do, regardless of the pleasure or displeasure it causes. Even if the displeasure is as great as death, the moral person does not hesitate to do his duty. He helped Arjuna face the displeasure of death by pointing out that life-energy is never destroyed. Like all forms of energy it is never lost, it merely changes form.
Now Krishna will begin another series of extremely important statements that strengthen and elaborate on the principle that life force is indestructible and eternal.
“Something which can be destroyed never really exists, and something that truly exists can never be destroyed,” Krishna explained. “People who can really see, see it that way.” 
Arjuna will ask, “What really exists?” So…
“The thing which pervades everything is what cannot be destroyed. It is impossible to destroy the indestructible,” Krishna explained. 
“The thing which pervades everything” is life-force, the soul. (a) Life force is everywhere in the universe, giving energy to matter. (b) the body, a microcosm of the universe is pervaded by the life-force of a specific soul.
Arjuna will now ask, “What doesn’t realy exist?” So…
“The body of the indestructible and infinite soul is destructible. Therefore fight!” 
Since the body is destructible it must end at some point. Therefore if the course of duty involves death, it should not cause one to abandon duty – death is inevitable anyway.
Can this philosophy support suicide bombers, etc.? Yes it can be misused in that way. Knowledge is dangerous. A little knowledge is even more dangerous. Just because knowledge can be misused doesn’t mean it should be altered or suppressed. It means that the misuse should be guarded against.
“Someone thinks he is a killer. Someone else thinks he is killed,” Krishna said. “Neither of them know anything. The soul cannot kill or be killed.” 
Nothing can be killed because nothing that is temporary actually exists. Phantasmagoria exists for some time only. That is why it is called “phantasmagoria.” This philosophy can be used to justify murder, so we should be careful to guard against those who would seek to exploit these words of knowledge to justify undutiful and immoral actions.
Next Krishna will quote / paraphrase the Kaṭha Upaniṣad (1.2.18) to provide Śastric support for what he has been saying so far.
“Never born; nor ever dying; Completely outside the scope of past, present and future; it is unborn, eternal, ever-existing, and primeval. It is not killed when the body dies.” 
“Arjuna,” Krishna continued, “you know that the soul is indestructible and eternal, unborn and undying. So why is it that you think you can hurt or kill anyone?”