For those apparently not up to speed, the war in Vietnam had been going on since 1945. By the 1950s, American military personnel were in country as military advisors to the South Vietnamese. Unless Askin's autobiography is a complete fabrication, Col. Askins was stationed there as chief instructor of firearms for the entire South Vietnamese Army.
He had arrived there with a battery of hunting guns, including one of the first .44 Magnums, and indulged in the pursuit of game at every opportunity. It was during one of these trips that he encountered the enemy soldier and killed him. Anyone is free to call this murder. My take is that Askins saw an opportunity to reduce the opposing forces by one and took it.