Mr. Laser Beam wrote:
Tracey has no excuse - he willingly and carefully decided to violate the Prime Directive. He suffered no trauma, no wrenching event that caused him to go cuckoos.
Admittedly we don't know how Tracey reacted to the death of his crew and what happened in the 6 months after that. When Kirk arrives he makes a rather sober report but doesn't show a real human emotion of concern or regret that he lives and his crew died (compare to Matt Decker).
From the start he is deceitful: Oh, I'm so glad you arrived Jim to stop this public execution (after he had previously killed hundreds of Yangs...).
The next time we see him in action is killing the injured Lt. Galloway (he could have just stunned him). After he had already lost his entire crew, I would have hoped he'd make sure that no further Starfleet personnel were killed "under his watch".
That he regards the Yangs as wild animals is one thing, but obviously his murder of Lt. Galloway reveals what kind of man he actually is. He'd sacrifice hundreds of lives just to save his own despicable skin.
IMHO, you could have marooned Kirk and Decker for 6 years on the planet and they still wouldn't have done the things Mr. Tracey did.