1/5 - Poor
Here was an episode that misfired on nearly every step. And, considering they (imo) captured the essence of 'Star Trek' with the first regular episode filemed - The Corbomite Manuever - I don't buy the 'it was an early episode' excuse as this was the 6th hour (counting the two pilots) produced.
In the long run, here we have an intelligent creature ('new life'); and the lat of it's kind; and while Dr. Crater is neglegent in trying to conceal the creature's nature; Kirk doesn't help by denying the salt request until he's 'satisfied'.; etc.
Hell, the creature (masquarading as McCoy in a briefing near the end of the episode) even brings up the fact to Kirk that the creature is intelligent and perhaps could be reasoned with and an accord struck; but at that point Kirk states in no uncertain terms that the creature needs o be killed.
And don't get me started on the final scene with Spock yelling at the top of his lungs, "Shoot! It's KILLING the Captain!" - yep good, un-emotional 'in character' writing for Mr. Spock there.
Again, overall, I find this to be the WORST episode (yes, even worse than The Alternative factor); in an otherwise, mostly stellar first season of TOS.
YMMV.