It appears it hasn't been mentioned, yet, but the entire handling of the Salt Vampire in "The Man Trap" must have been quite an ethical dilemma for Spock, at least in this episode’s aftermath, hopefully.
SALT VAMPIRE (disguised as McCoy): Oh. Well, we could offer it salt without tricks. There's no reason for it to attack us. (that’s the Salt Vampire disguised as McCoy!)
SPOCK: Your attitude is laudable, Doctor, but your reasoning is reckless.
CRATER: (eyeing McCoy carefully) The creature is not dangerous when fed.
SALT VAMPIRE: No, it's simply trying to survive by using its natural ability to take other forms.
CRATER: The way the chameleon uses its protective colouring, an ability retained no doubt from its primitive state, the way we have retained our incisor teeth. They were once fangs. Certain of our muscles were designed for chase. It uses its ability the way we would use our muscles and teeth if necessary, to stay alive.
SALT VAMPIRE: And like us, it's an intelligent animal. There's no need to hunt it down.
SPOCK: A very interesting hypothesis, Doctor
What’s happening in this episode’s climax is not to “seek out new life” but to trap, provoke and finally kill it. The Salt Vampire was undoubtedly under stress, irritated and confused.
They knew that they just talked with the creature, yet Spock doesn't come up with a contingency plan to prevent what eventually happened.
It always looked to me that the producers understood that this was not an episode to write home about.
In contrast we see in “Arena” quite a different Spock who almost appears eager to redeem himself for his colossal failure in “The Man Trap”.
SPOCK: You allude to invasion, Captain, yet positive proof
KIRK: I have all the proof I need on Cestus Three.
SPOCK: Not necessarily, sir. Several possible explanations…
KIRK: How can you explain a massacre like that? No, Mister Spock. The threat is clear and immediate. Invasion.
SPOCK: The destruction of the alien vessel will not help that colony, Jim.
KIRK: If the aliens go unpunished, they'll be back, attacking other Federation installations.
SPOCK: I merely suggested that a regard for sentient life…
KIRK: There's no time for that. It's a matter of policy. Out here, we're the only policemen around. And a crime has been committed. Do I make myself clear?
SPOCK: Very clear, Captain.