The ending is unrealistic and should have been rewritten
In real life, it's unlikely that an enraged person would have merely given his wife's murderer a clean execution. Those types of things tend to inspire brutality. It's more likely that Brad Pitt's character would have shot the suspect in the genitals or knees and then brutally assaulted him, leaving him maimed but probably alive.
I'm wondering why the director made such a huge oversight in his script.