If we look at the ocean, it is full of waves, mist, foam, along with a calm/deep section.

Even though the mist, waves, and foams seem different, they all are 100% water. They are just names given to various forms of water.

Waves may collide or change the form to mist or foam, but none of this makes any difference to water. Water remains unimpacted, unmoved, untouched by all these transitions and activities.

Similarly, countless animal/plant/human forms are born and die each day. Their bodies go back to earth and are recycled-reused. The jiva or Sukshma/Karan Sharir goes on taking new forms.

All these animal/plant/human forms also interact with each other. They fight, eat or destroy each other. They also interact with numerous non-living entities.

But, just like the water of the ocean, even though all these entities are 100% Brahman, Brahman remains unimpacted, unmoved, untouched by all these activities or changes.

And not to forget – We are that Brahman which stays “unimpacted, unmoved, untouched”.

Once we realize this true essence of ourselves, we will be able to stay unimpacted, unmoved, untouched by activities while being fully engaged in them.

Therefore, as Brahman, how can we kill someone? Or how can someone kill us? We all are ever unattached to the activities of the body. We just appear to be doing things as we identify ourselves with the body.

So, Shri Krishna says in next shloka:

य एनं वेत्ति हन्तारं यश्चैनं मन्यते हतम् |
उभौ तौ न विजानीतो नायं हन्ति न हन्यते || 19||
Those who think Brahman can kill someone or someone can kill Brahman are incorrect. In reality, Brahman neither kills nor can get killed.

Furthermore he adds that:

न जायते म्रियते वा कदाचि नायं भूत्वा भविता वा न भूय: |
अजो नित्य: शाश्वतोऽयं पुराणो न हन्यते हन्यमाने शरीरे || 20||
Brahman is never born and will never die. There was never a time when it didn’t exist, and they will never be a time when it will cease to exist. It is without birth, eternal, immortal, and ageless. It does not get destroyed with the body.

But we may ask, why is Krishna telling all this to Arjun?

If you remember, the main reason Arjun developed cold feet was that he couldn’t digest the fact that he has to kill Bhisma/Drona to win the war.

But, over the last ten Shlokas, Krishna has explained to Arjun that he is not just this body+mind. He, as all of us, is none other than Brahman. He is eternal, so are Bhishma/Drona. Arjun can not kill Bhishma/Drona, nor will they die.

It is the bond of Karma between their Sharirs that requires him to fight Bhisma/Drona. Also, as a Kshatriya, it is his duty to fight the धर्मयुद्ध.

Finally, there is no way he, identifying with the body, can run away from this. Even though he runs away now, he will have to fight them in the future, as no one can run away from their Prarabdha Karma.

All the Best!