You know how this would work in real life: having done exactly the right thing, the thing that had to be done, Kirk would be put on trial for murder.
"Why did you decide to follow Mr. Mitchell?"
KIRK: "Since I was unconscious when Dr. Dehner's powers manifested themselves, I went after her, not Mitchell. She may have only been on the ship a short time, but she was still a member of my crew. Unless absolutely necessary, I refuse to leave a crewman behind."
LAWYER: "And you don't think insuring the safety of the Federation and beyond was a necessary enough reason?"
KIRK: "You read the report. She was still in the early stages of metamorphosis. I was able to reach her humanity and she was instrumental in buying me the time to stop Mitchell."
LAWYER: "Says here that Mitchell regained his powers shortly thereafter."
KIRK: "She still weakened him enough for me to break free and attack."
LAWYER ACTIVATES VIEWSCREEN. THE FIGHT SCENE PLAYS.
LAWYER: "By hitting him in the jaw? In the stomach? Karate chops? Why didn't you simply run for the rifle?"
KIRK: "Now hold on a minute. My report wasn't that detailed. All I said was they gave their lives in the performance of their duties! Where did you get this video?"
LAWYER POINTS TO THE BACK OF THE ROOM. A SHORT, BALD, HUGE SKULLED ALIEN, WITH PULSATING HEAD VEINS, SITTING NEXT TO BEN FINNEY.
KIRK: "Damn it."