Question: Who's idea was it, or the reason, Kubrick decided to kill off Gunnery Sergeant Hartman? Was it to merely show the casualty of war?
Answer: It was Gustav Hasford's idea. It happened in the original book that the story is based on, "The Short Timers."
Answer: It was to show what someone might do (someone with a loaded gun and confronted by their abuser) whilst in the process of having a mental breakdown.
Question: What is the deal with Mac? Why isn't he in the Armed Services like a majority of the other men in England?
Chosen answer: He could have flat feet or a bad back or many other problems which don't really inconvenience him but make him ineligible for service.
Chosen answer: The first time, Jim says "Nanika no machigai desu. Bokutachi wa minna tomodachi desu yone. Senso no sei desu." This roughly translates to "There has been some sort of mistake. We are all friends aren't we? It is the war's fault." The second time, Jim only says "Nanika no machigai desu. Bokutachi wa minna..." and then gets pushed aside. "There has been a mistake, we are all...". I hope that helps.