Spock is an ASD character who realized he didn't have to mask any more once he got a commission. Kirk thinks it's funny so he doesn't check him.

Could be. Wouldn't have been the first or last time he was an asshole. Spock is an unreasonable dick for basically all of That Which Survives-

Rahda: We're holding warp eight point four, sir. If we can maintain it, our estimated time of arrival is eleven and one half solar hours.
Mr. Spock: Eleven point three three seven hours, Lieutenant. I wish you would be more precise.

Lt. Cmdr. Montgomery 'Scotty' Scott: Mr. Spock, the ship feels wrong.
Mr. Spock: "Feels," Mr. Scott?
Lt. Cmdr. Montgomery 'Scotty' Scott: I know it doesn't make sense. Instrumentation reads correct, but the feel is wrong. It's something I can't quite put into words.
Mr. Spock: That's is obvious, Mr. Scott.

Mr. Spock: In that case, Mr. Scott, I suggest we start at once. Can you give me warp 8?
Lt. Cmdr. Montgomery 'Scotty' Scott: Aye, sir. And maybe a wee bit more. I'll sit on the warp engines myself and nurse them.
Mr. Spock: That position, Mr. Scott, would not only be unavailing, but also... undignified.

Uhura: Mr. Spock, what are the chances of the captain and the others being alive?
Mr. Spock: Lieutenant, we are not engaged in gambling.

Dude gives the odds every time but that one. Presumably just to be a dick to Uhura. He was even a dick to M'Benga which is just not something I would recommend ever to anyone...

Dr. M'Benga: Well, the pattern of cellular disruption was the same, but as to the cause, well, your guess is as good as mine.
Mr. Spock: My guess, Doctor, would be valueless. I suggest we refrain from guessing and find some facts.

I mean why M'Benga didn't just break his neck right then and there...

Which is very in-character for him.