Kirk kills his best friend. Despite what we think we know about this episode, a lot of it is conjecture. Like the idea that they just started their mission. they didn't.
Captain's Log, stardate 1313.8: add to official losses Doctor Elizabeth Dehner - be it noted she gave her life in performance of her duty; Lieutenant Commander Gary Mitchell, same notation.
I want his service record to end that way; he didn't ask for what happened to him.
I felt for him, too.
I believe there's some hope for you after all, Mr. Spock.
-James Kirk and Spock