In the last scene, why did Macklin ... (SPOILERS!)

... shoot Fargo? Especially after his speech to Rennie about saving a life, not committing homicide. To me, it was out of character.

The actual last lines are:
Fargo: "Who are you supposed to be?"
Will Macklin: "Will Macklin. State traffic investigator."

but I was half expecting (and probably would have preferred) something like this:
Fargo: "Who are you supposed to be?"
Will Macklin: "Will Macklin. State traffic investigator. Two years ago, you mowed down my little sister in Idaho."


It was a hooker! It's not cheating if you pay for it!