Why do people complain that Back to the Future II is too dark?
Is it because of the alternate (It's a Wonderful Life inspired)-1985 version of Hill Valley under Biff's control, where George McFly is murdered?
https://www.youtube.com/watch?v=EqlmMw108rE
The first movie had its own dark moments too like Doc Brown being killed by the Libyans or the whole subplot of Marty having to preserve his and his siblings own existence while in 1955. I always found the scene in which Marty's hand starts to fade when he can't play the guitar at the dance to be extremely disturbing.