Know for certain that That which pervades all is imperishable. None can cause the destruction of That, the Imperishable. (Bhagavad Gita 2.17)
Only the material bodies of the Self are subject to destruction, it is said, while the Self itself is indestructible, immeasurable and eternal. Fight therefore, O descendant of Bharata. (2.18)
The basic thing to understand: ‘I am the body’ is a false idea. The bodies come and go, they change form, they each have a beginning and an end. The body you have now is entirely different from the body you had when you were a newborn baby. Therefore identifying oneself with the body is an illusive idea. The real Self, of which the body is just a tool, is indestructible, immeasurable and eternal.
Here is a quote from Ramakrishna Paramahamsa that helps us understand the Self by which all is pervaded.
Take the case of the infinite ocean. There is no limit to its water. Suppose a pot is immersed in it: there is water both inside and outside the pot. The jnani sees that both inside and outside there is nothing but Paramatman. Then what is this pot? It is ‘I-consciousness’. Because of the pot the water appears to be divided into two parts; because of the pot you seem to perceive an inside and an outside. One feels that way as long as this pot of ‘I’ exists. When the ‘I’ disappears, what is remains. That cannot be described in words. (The Gospel of Sri Ramakrishna, p. 915)
Even the idea that the material body undergoes destruction is such a false idea. Matter doesn’t come out of nothing. We know that energy can neither be created nor be destroyed, it only changes from one form to another. Similarly, the basic particle of matter, call it ‘atom’ if you want, can neither be created, nor be destroyed. The particles only combine and recombine with each other to change forms. Even after the death of the physical body, the particles that made up the body don’t go away. They just disperse and recombine with other particles to form or feed up other bodies. So what is born and dies is just the idea of a particular form and the name you attach to the form. Take away the name and form and the whole idea of birth and death, creation and destruction, goes into pieces.
One who takes the Self to be the slayer and the one who thinks He is slain, neither of them knows; The Self slays not nor is He slain. (Bhagavad Gita 2.19)
This (the Self) is never born, nor does it die at any time. This has never come into being, never comes and never will come into being. This is eternal, permanent, the most ancient, is not killed when the body is being killed. (2.20)
One who knows this Self to be indestructible, eternal, unborn and immutable, O Partha, how can that person slay or cause to be slain. (2.21)
Also, the idea of separation exists only when we identify with this body, a particular name and form. We say ‘I’ and ‘the other’ only with respect to name and form. When we understand that the identification with name and form is just an illusion, there is no more separation, there is only One. Then there is no more selfishness, there is no harming somebody else to do good for yourself. All ideas of fear, hope, worry, anxiety, desire, craving, killing, being killed, all these ideas vanish. You know yourself to be the One Self, you are the only one. With this knowledge, you live in the present and do the most appropriate thing you can do at this moment.