What's up with oung people getting killed in 60's-70' movies

As mentioned by earlier posters, it was very much a product of the culture, which came down to "Us vs. Them"--namely, the baby boom generation vs. "the Man" (the establishment, authority figures, the government, organized religion, or just about anyone from an older generation). The baby boomers were beginning to reach adolescence in the late 1950s and early 1960s, a time of great optimism and promise for the U.S. Donald Fagen's 1982 song "I.G.Y. (What a Beautiful World)" reflects the feeling of a lot of people in the U.S. during that period. The election of President Kennedy in 1960, a relatively youthful President with two very young children, and the aura of a higher, more sophisticated culture that his wife Jackie projected, had a huge appeal for a lot of young people in those days.

When Kennedy was assassinated, it was a hugely traumatic event for the country and for younger people in particular; the trauma continued for years until it was surpassed by the events of 9/11. Anyone under the age of 25 wouldn't remember the years of mourning that went on for JFK; the assassination and the hordes of conspiracy theories about his death were rehashed over and over again, often turning Kennedy into an almost Christlike figure who died for all our sins--Oliver Stone's JFK sometimes plays like a religious epic about the Crucifixion, IMO.

The escalation of the Vietnam War, fought predominantly by armed forces consisting of teenagers and early twentysomethings, added to the feeling that the older generation was out to get the younger ones. Bonnie and Clyde depicted the title couple as idealistic, fun-loving young people who just wanted to get back at the authorities (the evil, greedy banks) without doing any harm to anyone (ignoring the reality of the real Bonnie and Clyde, who were lowlife, sadistic scum); Clyde getting a piece of his head blown off in the final scene was an intentional reference to the Zapruder film of Kennedy's assassination, a film which was replayed so often in news shows and documentaries back then that most baby boomers knew it by heart.

The war continued into the 1970s, until it simply petered out, so there was no real sense that all those who died had achieved any real victory, and people were embittered and cynical about that, too, which was reflected in the movies. The blockbuster era came along in the mid-'70s and Hollywood decided that these were more profitable than the nihilistic movies of previous years. 

Donald Fagen's The Nightfly is a great album, by the way.