• Mike Jackson, a British Army officer participating in the NATO intervention in Yugoslavia in the 1990s, refused to obey orders to capture a Russian-held airport (which could have sparked direct conflict between NATO troops and the Russian Army):
  • Mike Jackson served in the NATO chain of command, reporting to the Supreme Allied Commander Europe, American four-star General Wesley Clark. Under Jackson's command, the ARRC deployed to Bosnia and Herzegovina in March 1999,^[13] where Jackson served his second tour of duty in the Balkans, commanding KFOR, NATO's multi-national peacekeeping force established at the end of the Kosovo War. He gained significant media attention in June 1999 after a confrontation with Clark in which he refused to block the runways of the Russian-occupied Pristina Airport and isolate the Russian troops there, thus preventing them from flying in reinforcements.^[40][41] In one heated discussion with Clark, Jackson reputedly told him "I'm not going to start the Third World War for you".^[40] He later told the BBC he believed that obeying the order would have led to the possibility of an armed confrontation with Russian troops, which he felt was not "the right way to start off a relationship with Russians".
• By the same token, there are probably a whole lot of unsung heroes right now who are working to make sure that Russia/Ukraine/NATO are able to navigate as best as possible this scary new phenomenon of fighting a massive, bloody, tragic, near-total, yet still limited war over the future of Ukraine, without escalating into a potentially nuclear direct conflict between Russia and NATO.  Most notably, Ukraine holding back from attacking targets inside Russian territory and Russia holding back from attacking targets inside NATO territory, even though it must be extremely tempting to do so.
• There are definitely lots of heroic stories from WW2 where commanders went out of their way to protect civilians, or refused to destroy towns / cultural landmarks even when it would've been tactically advantageous (taking Kyoto off the list of atom-bomb targets is a famous example, although there doesn't seem to have been much pressure to keep Kyoto on the list -- there have been many smaller-scale showdowns where the pressure to attack a treasured cultural site must have been much higher), or refused to cooperate with Nazi authorities who were rounding up Jews and others as part of the Holocaust.  (Same goes for people who refused to cooperate with other oppressive police states, such as the east-German Stasi or Soviet KGB).  Unfortunately I am not familiar with many specific stories.
• It would be interesting if anyone could name sufficiently large-scale examples of a corporation making a strongly virtuous decision against a strong profit motive.  Ie, a cigarette / gambling / tobacco company deciding to wind down their activities or change their business strategy (in a way that actually helped long-term, rather than just creating an opening for someone less scrupulous), or lobby for regulations that would hurt them but serve the public interest, or etc.
• Similarly, it would be interesting to hear examples of people inside non-military government agencies who made important decisions that went against their personal incentives.  There are many Watergate-like stories of law-enforcement agencies investigating corruption even though higher-ups want to shut down the investigations.  I could also imagine there might be some heroic stories along the lines of someone at the FDA approving an experimental medicine for, say, HIV, despite public disapproval of homosexuality at the time and the notorious culture of risk-averse safetyism at public health agencies.  (The first confirmed cases of "community spread" of Covid-19 were discovered by intrepid researchers breaking CDC orders that mandated who was allowed to be tested and what tests were allowed to be used!)

Not quite as dramatic as a commander disobeying military orders, but a similar category includes basically all "could-be dictators" who could've assumed absolute control and used it for selfish ends, but instead worked to create a better society and then willingly relinquished power at the end of their terms.  These people might not have been under external pressure to continue ruling, but they probably resisted strong personal temptation and the pressure of their supporters/allies:

• George Washington refusing to run for a third term in office in 1796, setting the precedent of the two-term US presidency.
• (Probably a whole lot of George-Washington-like figures in post-colonial countries in Latin America / Asia / Africa that gained their independence and adopted democracy over the past 200 years!  Unfortunately I can't name many detailed examples.)
• Juan Carlos De Borbon was the King of Spain in the 1970s, pretended to be supportive of Fransisco Franco's fascist government, and due to that loyalty he was hand-picked by Franco to succeed him in leading Spain's autocracy after Franco's death.  But actually, shortly after Franco died, Juan Carlos revealed his true colors, pivoting towards enthusiastically embracing democratic reforms and shepherding Spain through a bloodless transition to democracy!!  He later abdicated the throne in 2014, ending the monarchy.  This story has always been a huge moral inspiration to me.
• Mikhail Gorbechev helping dismantle the Soviet Union.  (To a lesser extent, you could count Deng Xiaoping in China who pivoted China towards capitalism and away from the insane chaos of the Mao era.  To an even lesser extent, you could count Khrushchev in the Soviet Union, who "de-Stalinized" the USSR but otherwise kept the oppressive communist system going.)
• Al Gore notably decided to respect the (controversial) decision of the Supreme Court after the ridiculously-close presidential election of 2000, refusing to go full-steam and litigate the election indefinitely out of respect for the nation's civic integrity.
• The "Ibrahim Prize" is a Nobel-Prize-like award given out "to a former African executive head of state or government on criteria of good governance, democratic election and respect of terms limits" -- basically for people who make like George Washington and do the right thing at the end of their term.  It has been awarded seven times since 2007, including an honorary retrospective award to Nelson Mandela, and most recently to Mahamadou Issoufou in 2020 after the first-ever peaceful democratic transition of power in Niger.

This is much smaller and much more recent, but Li Wenliang seems to have been somewhat idolized for sharing information about COVID in December 2019. At that point the local government policy on COVID was roughly similar to what policy had been on the SARS epidemic: keep it quiet and don't let people know what is happening. Local police/government officials were not happy that he shared this information. He died from COVID in February 2020. I haven't crunched the numbers, but it seems reasonable that X number of severe illnesses and Y number of deaths ended up not happening because he shared information about COVID.

Hugh Thompson Jr. ended the Mỹ Lai massacre by instructing their helicopter crew to fire on their own military's soldiers if they continued to kill innocent civilians, then informed command of what was going on and got them to order the company committing the massacre to stop.

All sorts of people helped Jews escape the Holocaust at their own risk. Oskar Schindler, for example, was originally a member of the Nazi party, then saw what was going on and spent their entire fortune on bribes to keep their Jewish employees from being sent to concentration camps. They pivoted their business from manufacturing cookware to manufacturing anti-tank grenades (and tried to manufacture as few as possible) in order to get those employees classified as "vital to the war effort". They went bankrupt and had to live much of the rest of their life off of financial aid from the Jews they had saved.

There have been a number of other nuclear close calls that may well have been prevented by a single person, but I don't know the details.

Maybe Georges Picquart for their rule in the Dreyfus affair. Kolmogorov sheltering Jews. Jimmy Carter trying to end racial discrimination against the desires of their constituents.

Not nearly as large in scope as Petrov or Arkhipov, but John Rabe was a German businessman and head of the Nazi party in Nanjing. He was responsible for creating the Nanking Safety Zone during the Nanjing Massacre (also known as the Rape of Nanking). Estimates suggest that he sheltered around 250,000 civilians from being killed in one of the most brutal atrocities of WWII. 

Swedish architect and diplomat Raoul Wallenberg issued and distributed countless illegal "protective passports" to Hungarian Jews awaiting to be deported to Auschwitz, often defying orders from Nazi officials and risking his own life. A witness recounted a relevant incident as follows:

[Wallenberg] climbed up on the roof of the train and began handing in protective passes through the doors which were not yet sealed. He ignored orders from the Germans for him to get down, then the Arrow Cross men began shooting and shouting at him to go away. He ignored them and calmly continued handing out passports to the hands that were reaching out for them. I believe the Arrow Cross men deliberately aimed over his head, as not one shot hit him, which would have been impossible otherwise. I think this is what they did because they were so impressed by his courage. After Wallenberg had handed over the last of the passports he ordered all those who had one to leave the train and walk to the caravan of cars parked nearby, all marked in Swedish colours. I don't remember exactly how many, but he saved dozens off that train, and the Germans and Arrow Cross were so dumbfounded they let him get away with it.

Estimates vary, but Wallenberg may have saved between 4,500 and tens of thousands of Jews toward the end of World War 2.

Maybe Vavilov and his colleagues?

(Have not fact checked this account, but the Wikipedia page seems to broadly agree)

Definitely not a hero all-things-considered, but Albert Speer refused to obey Hitler's Nero Decree of March 19, 1945, which ordered the destruction of "All military transport and communication facilities, industrial establishments and supply depots, as well as anything else of value within Reich territory that could in any way be used by the enemy immediately or within the foreseeable future for the prosecution of the war."

Sir Robert Lambert Baynes and his refusal to escalate the War of the Pig ... but in fairness that one would probably not have spiraled out of control in any case.

On the corporate front, the only example I can think of is SC Johnson's choice to discontinue the use of PDVC in manufacturing Saran Wrap. It could be argued that this choice was just sensible risk management in response to FDA warnings; but the CEO, Fisk Johnson, seemed sincere enough in offering his rationale for the decision ("The goodwill of people is the only enduring thing in any business. The rest is shadow,").

Frances Oldham Kelsey also deserves a shoutout for more or less single-handedly preventing a hundred thousand or more birth defects despite considerable corporate pressure. 
 

Not a perfect answer but why the focus on pressure towards making a single negative decision? 
Here are two (of many more) assassination attempts on Hitler. While not successful, they were probably nevertheless very high EV. 
1) Georg Elser placed a time bomb at the Bürgerbräukeller in Munich, where Hitler was due to give a speech in 1939. Hitler's flight was canceled due to bad weather so he left early & before the bomb detonated. Elser was in response held as a prisoner for over five years and eventually executed by the Nazis at the Dachau concentration camp.
2) In 1944, Claus von Stauffenberg along with many other conspirators tried to kill Hitler in a bigger & rather well-known plot called Operation Valkyrie. In the plot, they aimed to overthrow the nazi government and (most likely) as quickly as possible make peace with the Western Allies. Due to several coincidences, Hitler was only weakly injured. Stauffenberg was killed quickly afterward.