Betty in Mad Men - just no need, it was the end of the series and unrealistic for a young woman to die of lung cancer
Cordelia in Angel - think they did this to get back at the actress for getting pregnant, the death was so senseless and ruined the long term story arc
Mark in ER - could he not have had a happy ending on that beach? Also it was such a rare condition.
Meg in Home and Away - this made teenage death seem so real
Chris in Stand By Me - a bad boy made good surely he was due good karma and a long life?
Luke and Han in Star Wars - just done for shock value
No one ever dies of dementia in films or tv!