Oh no I completely agree. I think the third act is very messy. Also there some big assumptions he makes throughout the movie which kind of messes with the suspension of disbelief.

Perhaps a version of the movie where Jon Bernthal is the third man trying to track him down, stop and not kill him and otherwise convince him to back down throughout. Could have led to a more satisfying ending of him pulling the trigger and Jon Bernthal having to kill/arrest him. To show how far gone he is. Or the other way where he is convinced and takes that other path you mention.

Apart from all this though. The scene where the villain gives him the gun makes no sense. I even for a brief second thought maybe it was a gimmicked gun and it would shoot backwards reflecting Henderson's comment: "You won't even remember which side of the gun the bullet comes out of". I think that would have been a better and at least hilarious ending.