This is true to the actual wild west than most westerns. Bravo to Mortensen for telling a true to life story.
(If you are looking for the typical rich guy is the good guy BS western instead of reality, you'll be sorely disappointed.)

The acting and directing is fantastic, but this need to go back and forth constantly throughout a timeline becomes tedious to follow along. In this case, it becomes more of a distraction, particularly in the beginning of the film. The viewer is left guessing which part of the timeline they are "in" and frankly it's more work than it's worth. Starting at the almost end and then going straight through from the beginning to the end would've been better.
Normally I would subtract 2-3 points from the score just from the tedious back and forth. It's only necessary at minimums and should be avoided except as necessary. This was a bit too much.

However, the story is compelling. So I've marked it a 9/10, but really feel that's the score for the story itself, the overall script albeit not the editing of the presentation. It's probably only a 7 if the constant back and forth becomes too annoying for the viewer. I'm grading the story, the acting and not the one editing mistake.