Why is it in most all countries you get imprisoned for killing someone and yet when governments go to war no one that kills gets prosecuted

That's not always true. Nazis were tried for, and many convicted of, crimes against humanity at the Nuremberg trials. 

Not really a factual statement. 

A. There are rules to war, you kill people by the rules  you do not get prosecuted.  If you mess up you may be prosecuted. 

B. I can think of a number of places I have been to or know about where killing someone is just overlooked by the law.  If the "Law" wants you then you will be prosecuted.  Simple.

C. Different rules for murder depending on who commits it.  I can think of at least six locales where this holds true; Sudan, Zimbabwe and South Africa are high on these lists.  Nigeria and Mali are the most wide open in what constitutes murder as opposed to just killing.  Dominican Republic if you are a Tourist your Murder and Rape may not be listed as such by the police.  And the list goes on and on.

Zimbabwe easiest place to dispose of Body, followed by Sudan and quite possibly Afghanistan.  Now if you do not follow protocols murder and sloppy disposing of bodies may land you in a Clan Feud in Afghanistan or Baluchistan.

Homicide is a legal term for any killing of a human being by another human being. Homicide itself is not necessarily a crime.

Murder is the intentional killing of another human being.

War is state sanctioned murder and is legal.