Thoughts... (SPOILERS!)

it said it's based on true events though.

and at the end he didn't kill himself. so he could have gone to jail and told his side of the story. he obviously was a nut job. yes people fly off the handle when they are cheated on. especially walking in on it---on your anniversary. very traumatic and lots of people, the average person would flip their shi* and probably yell and scream even maybe say out loud "im going to kill you." but this guy immediately murdered them. he saw them, walked downstairs, got the guns, and shot them. it wasn't clear on where/when he was having the elaborate and detailed fantasy. and that def could have been embellished for dramatic purposes for the film. but its possible he had a bit of a detailed fantasy/episode. not THAT eleborate. but maybe just the peices at the end, the names he had conjured from the day. someone who is able to just immediately kill two people without hesitation and have the kind of rage he obviously did---- is definitely able to have some kind of a mental fantasy like that if he spun out of control. he was definitely crazy. so its not too far of a reach.

i think your idea for a twist is cool--- but again it was based on a true story of some couple out there......so they prob wanted to keep it kinda close to its original events. or else why say at the beginning of the film "based on true events" only to totally movie it out and make a whole diff twist than the real story just so it makes more sense in our eyes.

i was a bit disappointed and put off too that the ENTIRE THING was in his mind because as someone not crazy i cant relate to something like that but its for sure possible.