It was that Chapman guy. He was another Catcher in the Rye fanatic who decided to shoot lennon. Guy had a history of being disturbed.
Reagan's failed assassin was the one who wanted Foster's attention.
As to Kurt Cobain, I'd say the movement wouldn't have lasted much longer than it did. There were still plenty of Grunge acts around. Famous ones like Pearl Jam, even. (who through principle and hubris stunted their own careers, even though in a sense they were right) The Grunge movement was sort of destined to burn out. Besides, Kurt's death became a legacy. His death kind of energized a few years worth of generations. Plenty of kids were inspired to become musicians because of the whole ordeal, but they adopted other musical styles.
Better yet, Dave Grohl survived the transition and is still putting out music.