vedaavinaashinam nityam ya enamajamavyayam |
katham sa purushaha paartha kam ghaatayati hanti kam || 21 ||

Whosoever knows this (eternal essence) to be imperishable, eternal, birthless and changeless, how can that individual, O Paartha, slay or cause anyone to be slain?

veda : knows
avinaashinam : imperishable
nityam : eternal
yaha : whosoever
enam : this
ajam : birthless
avyayam: changeless
katham : how
saha : that
purushaha : individual
paartha : O Paartha
kam : who
ghaatayati  : cause to be slain
hanti : kill
kam : who

Imagine that you are operating a laptop that is connected to a printer. You open a document on the laptop and click the print button. The laptop sends a signal to the printer, and the document gets printed.

Now imagine that the laptop and the printer have egos and can think for themselves. The laptop will say “I initiated the printing action” whereas the printer will say “I was the receiver of the printing action”.

But in reality, an electric current went from the laptop to the printer, which then caused the printing to happen. So, an engineer would never say “the laptop caused the printing to happen”. It was all a play of electricity.

So here, what Shri Krishna is trying to say is that “this”, the eternal essence does not act and nor does it get impacted by any action, just like electricity does not really act. It empowers material objects to act without actually acting. Therefore a wise person will never think that the eternal essence can slay or kill or act at all. It is like the sun.

The refrain is clear: “You are the eternal essence – birthless, changeless, eternal and imperishable. The eternal essence does not slay, nor does it get slain.” It is repeated throughout the Gita because it will take a significant effort on our part to truly understand and internalize this message.