Bhagavad Gita2

“There was never a time when I did not exist or you did not exist, or these kings did not exist;                                                                  
and there will never be a time when all of us will not exist.”

Krishna telling Arjuna, you are only going to kill the form. The form is temporary but the soul is eternal. The soul was there before as well as after the form.. Humans live with time and space but for god there is no such term as time or space. We humans divide our world into Past, Present and Future. For God the entire functioning of the world as a single event (ie) Present.

God sees the entire forms of life of a Soul as a single event. But Arjuna is a human and he was directed by mind. The mind is always corrupted by bringing the concept of time and confusing him.
What Krishna emphasized on this Sutra
You cannot kill something which existed before and also appear in the future. 
You cannot create something which never existed before but to appear in the future. 

To understand simple, Portuguese Explorer Vasco de Gama discovered India. Actually India was there before and also after Vasco de Gama. Actually he has discovered India for himself and not for the world because India existed even before and after his visit. (The meaning of real discovery is when you find a place which was not inhibited by humans and never known to the world where such a place exists.)

Krishna advises Arjuna to take part in the war. If you are not killing them they are going to kill you. Suppose you want the reason ahead of killing them. I can say they were never here in this form 100 years before and won’t be there 100 years after. By killing them you are just giving them an early exit. 

Krishna knows war will eliminate the evil and weaker humans to replenish with new and healthy one like how the recession eliminates the weaker entrepreneurs. Why Krishna takes so much effort to give reason because we humans live by mind. Mind always wants a reason for everything. If someone helps, If you find it irrational then you will have a doubt and look for reason.