I never thought about this until reading it somewhere on this BBS I think . . . they could've gone Horta-esque and just fed it lots of salt and not killed it, right?
I know it had killed, but it was starving; they later forgave MamaHorta and moved on, right?
Come to think, why didn't it just go to the food slots or the galley and eat a bunch o' salt? Eh, whatever, I like the ep's early moodiness and conversational dialog. I generally prefer the vibe of the earlier eps and wish they'd kept it.
But they didn't. So we we move on, boats against the current, borne back ceaselessly into the late '60s.
I think the Salt Monster had them backed into a corner so McCoy had no choice but to phaser it to save Kirk.
Based on other episodes they probably would have saved it if Spock had a choice (unless it was killing the Captain - then no dice)