In Star Trek Into Darkness, Spock reported Kirk's violation of the Prime Directive. Meaning, Spock's character is unchanged in new timeline; he still strictly follows rules.
Later in the movie, Pike scolded Kirk that their mission was to observe, NOT to stop that volcano. If that's the case, why did Spock do the opposite?
Surely, following your captain's orders is within the rules. When Spock participated in the volcano mission, he was acting under Kirk's orders. The way I understand it, this is also the reason Spock did not receive the same punishment as Kirk. Ultimately, a captain is responsible for the actions of his crew.
Furthermore, as pointed out in the comments, Spock proceeds to be honest in his report to Starfleet, staying true to his nature, whereas Kirk attempts to deceive his superiors – which is pointed out by Admiral Pike as the major offence.
Spock did not want to violate the Prime Directive, which states (emphasis mine):
As the right of each sentient species to live in accordance with its normal cultural evolution is considered sacred, no Star Fleet personnel may interfere with the normal and healthy development of alien life and culture. Such interference includes introducing superior knowledge, strength, or technology to a world whose society is incapable of handling such advantages wisely. Star Fleet personnel may not violate this Prime Directive, even to save their lives and/or their ship, unless they are acting to right an earlier violation or an accidental contamination of said culture. This directive takes precedence over any and all other considerations, and carries with it the highest moral obligation
The volcano was about to kill everyone on the planet, so as long as they kept hidden and did not show themselves and their superior technology to the indigenous people on the planet, the prime directive was not being violated.
Pike on the other hand, scolded Kirk for not following orders, which were to observe the planet and their life forms, not interfere in any way. There is a fine line here, which Spock seems to be fine walking in.
While the other answers are all partially correct, they're missing a big part of the story: Spock had a death wish.
After the events of the first movie, Spock had trouble dealing with his grief and started putting himself at risk as a coping mechanism. This is explained in the prequel comics, Countdown to Darkness:
This culminates in Spock going into the volcano, as well as Spock and Uhura's emotional talk in Into Darkness.
The comic also explains Kirk's growing dismissal of the prime directive, which also plays a part in why Spock is in the volcano:
Here are some links on other people talking about Spock's death wish:
http://www.disgruntledindividual.com/2013/05/analysis-star-trek-countdown-to.html http://www.tor.com/2013/05/20/star-trek-into-darkness-review-spoilers/ http://fanlore.org/wiki/Spock/Uhura_(AOS)#Star_Trek_Into_Darkness http://allscaletrek.com/viewtopic.php?f=16&t=791
The key is that violating the Prime Directive is not a crime if it can be properly justified.
It's permitted for a Starfleet captain to violate the Prime Directive at his discretion. The subsection "Scope and interpretation" in Memory Alpha's article on the Prime Directive lists many examples, and states that:
If a decision was made by the commanding officer that could potentially be a violation of the Prime Directive, the conclusions and rationale would need to be recorded and justified to Starfleet through the ship's or station's logs.
It follows that crew members are absolved of responsibility in any Prime Directive violations approved by their captain. The captain makes the decision, the crew members register their objections, and then everyone follows the captain's orders. Otherwise, either you'd be punishing Starfleet personnel for following potentially legitimate orders, or you'd have to give any crew member the ability to disregard orders whenever they personally felt like a PD violation was involved; which might render the ship critically understaffed in a situation where that could least be afforded. The only way to resolve this dilemma is to say that the captain's word is law where PD violations are concerned, and on his head alone be it if he's wrong.
According to the film's novelisation, Spock did indeed have misgivings about the mission but those were overridden by his own sense of morality and the fact that he'd already hashed out the arguments with Kirk earlier (off-camera). A further refusal could be construed as mutiny in the face of a direct order, especially since the Captain has explicitly told him that the Prime Directive isn't being thwarted.
Tilting his head slightly to one side, he [Spock] spoke toward the suit’s pickup.
“Captain, did any representatives of the indigenous intelligence see you? At the risk of repeating the obvious and despite the difficulties inherent in our current effort, I must repeat that the Prime Directive clearly states that there can be no perceived external interference with the internal development of an alien civiliza—”
Despite the shuttle’s increasingly violent rocking, Kirk’s response came through clearly. “No, Mr. Spock, they did not! I know what it says! I might have missed a few details here and there in certain classes . . .” The admirable clarity of the surface-to-shuttle transmission was confirmed as Kirk’s communicator picked up the nearby McCoy’s unmistakable sarcastic snicker. “. . . but I didn’t miss that one. We’re not supposed to be here at all. It’s because of the Prime Directive that we’re having to do this the hard way. Now, drop off your super ice cube and let’s get out of here! Kirk out!”
Then shortly afterwards;
The science officer would have argued further with his captain save for two reasons: The time to do so had long since expired, and arguing with James T. Kirk frequently generated far more frustration than satisfaction. Filing the details of their brief conversation for future discussion, Spock returned his focus to the business at hand.
Up to that point, Kirk arguably hadn't broken the Directive. He'd certainly distorted it beyond recognition, but nothing he'd done could be construed as a direct violation. A few minutes later when he reveals the Enterprise, that's the biggie and something Spock had no part in agreeing.
“Congratulations, Spock. You just saved the world.”
“Captain. You violated the Prime Directive.”
“So they saw us.” The commanding officer of the Enterprise shrugged. “Big deal.”
Before the science officer could respond further, Kirk signaled to the members of the emergency response team. Any further deprecating comments disappeared beneath a whoosh of coolant gas and sprayed decontaminant.
