u/kevro

I can think of three:

Alien 3

Killing off Hicks and Newt off screen after all Ripley went through during Alien 2 was very unnecessary.

Terminator Dark fate, killing off John Conner instantly made the first two classic films pointless. All that plot and effort just for a very short scene of, well he get's him.

This one's going to be controversial but I loved the ending of Evil Dead II, just for it to be ignored in Army of Darkness. It left a bad taste in my mouth for the rest of the film. The best one is II.