You said it in your second sentence, disappointed.

It almost felt like they knew how they wanted the movie to end but didn't have a beginning or middle. So they found a script for a thriller about a young couple falling in love, however the guy started doing some creepy shit like stalking and eventually starts killing. The only thing with this script was that it didn't have an ending.

So what do you do when you have one script with a beginning and middle, and another script with only and ending? Well you mash them together and get Halloween Ends.