Why do the soldiers tell jokes about the war, about killing?

While it is true that many of the men are young and inexperienced in war, it is also true that they are trying to act more adult and grown up. Their use of profanity, shows that they try to act more like "men" even though it is hard in such a trying and frightening situation. As for their use of humor and joking about war and killing, people often make fun of those things which, if thought about too closely, would drive people to complete insanity. There is nothing funny about war or killing; it is their job, however, and the humor, such as it is, is a way for them to cope with what they are experiencing.