Supernatural's Sam and Dean Winchester faced many decisions throughout the 15 seasons, but didn't always make the right call. Granted, Sam and Dean's actions were sometimes in favor of the greater good, such as Dean ingesting phoenix ashes to kill Eve or Sam rallying hunters to take out the British Men Of Letters when they were eradicating the American hunter population. However, the brother's seemingly immovable dedication to cheating death in order to save each other often led to major ramifications on the plot.
Without staying steadfast in and prioritizing their original goal of "saving people, hunting things," they would not have made some of the choices they did either due to selfish reasons, like Dean resurrecting Sam to avoid being alone, or without considering the cruelty, like when The Winchesters tricked Jack into confining himself in a Ma'lak Box. So while they often went into situations with their intentions justified to them, Sam and Dean have made many mistakes in Supernatural that sometimes had ripple effects.
10 Dean Hunted Even After He Started A “Normal Life”
After Sam went to hell at the end of season 5, Dean returned to his old fling, Lisa, and her son, Ben, and attempted to have a regular life, bidding farewell to hunting. He even retired his beloved Impala during that time. Despite his grief and paranoia from a life of monster-hunting, Dean was relatively happy having a family. However, shortly after Sam showed up, Dean slowly slipped back into hunting. Ultimately, this led to one of his fears, namely Lisa and Ben in danger. Crowley's demons kidnapped them, one of whom possessed Lisa and stabbed her, nearly fatally. Without Castiel, she would've died, arguably because Dean didn't steer clear of the hunting life as he had intended.
9 Sam Trusted Ruby The Demon
One of Sam's most problematic decisions was trusting Ruby, even after discovering she was a demon. Unlike some other choices, Sam's decision to follow Ruby's lead was continuous, despite itting his reservations about her. Additionally, Sam and Ruby were never friends, but instead, their volatile relationship slowly led to Sam's demon blood addiction. This was something he was conflicted about and ashamed of since it was one of the many things he kept from Dean.
To make matters worse, Sam stopped Dean from killing Ruby in season 4, episode 21. Had Dean killed her then, Sam would not have freed Lilith, necessarily, which would have prevented Lucifer from escaping hell. If the seals had been left as is, the destruction and chaos Lucifer caused once he was topside wouldn't have happened. A key example of this is the Four Horsemen and the numerous innocent people who died because of their carrying out of risen Lucifer's instructions.
8 Dean Started The Apocalypse
In a similar light to the consequences of Sam trusting Ruby, Dean's decision to resurrect Sam at the end of season 2, regardless of what could go wrong, started the journey to the Lucifer-led apocalypse. In order to bring Sam back to life after one of Azazel's psychics killed him, Dean made a crossroads deal, ultimately g his death sentence and ensuring his soul went to hell.
By the end of season 3, Dean's time was up, despite Ruby's lies to Sam about saving Dean. This set up space for the seals to be broken. One of the 66 seals to free Lucifer happened to be a righteous man shedding blood in hell. As Dean confessed to Sam in season 4, episode 10, he withstood agony for 30 years before succumbing to Alastair and inflicting torture on souls brought to hell. This ensured the first seal broke, therefore making it easier to free Lucifer.
7 Dean Accepted The Mark Of Cain
While Dean's motive for getting the Mark was rooted in good intentions, it was quite rash and had seasons-long repercussions. Even with knowing the Mark comes with great burden, Dean didn't ask for further explanation, but instead took it on in order to wield the First Blade. His fixation on killing Abaddon above all else was reckless even though it somewhat aligned with his usual 'shoot first, ask questions later' mentality.
Because of the Mark, Dean battled with murderous rage and even exhibited pleasure in killing, like when he, Sam, and Jody cleared a nest of vampires. The Mark was also another factor that led to more lies between the brothers, such as Dean leading Sam to an unrelated location in order to kill Abaddon. Additionally, his Blade-induced rage caused him to attack Gadreel even when he was endangering himself by betraying Metatron to help the Winchesters. Probably one of the saddest effects of the Mark and the First Blade was that the fury and strength it gave Dean led to him almost killing Castiel.
6 Sam And Dean Resurrected Abaddon
Although the Mark of Cain was a horrible mistake, it wouldn't have even been an idea if Sam and Dean hadn't resurrected one of the most powerful demons in the show, Abaddon, to try and complete the trials that could close the Gates of Hell. The final trial involved curing a demon, and the brothers chose Abaddon, who was chopped up and buried at this point, immediately re-introducing her as a threat.
While Crowley had warned all demons to steer clear of the Winchesters in an attempt to prevent them from completing the trials, the brothers' carelessness by leaving the room after resurrecting Abaddon made them direct causes of the havoc she brought. Not only did this spark her vengeful mission to take over hell but also all the pain she caused, like using hunters as bait to get to Sam and Dean and voiding Crowley's demon deals, therefore bringing souls to hell prematurely.
5 Dean Stopped Sam From Closing The Gates Of Hell
After all the grief and struggles to complete the first two trials, Dean halting the very thing he advocated for previously was disappointing. A heartbreaking moment here included one of Supernatural's saddest deaths, namely Dean killing Benny in order to get Sam (and Bobby's soul) from Purgatory. Moreover, it's yet another indication that Dean's rash decisions often led to detrimental collateral damage whereby Sam and Dean's loved ones died.
Instead of shutting the Gates of Hell and significantly diminishing Earth's supernatural threats, Dean chose to save Sam's life even though he was willing to make that sacrifice for the greater good. Additionally, it could have meant Crowley would be human again and may have even ensured Abaddon was trapped in hell, unable to cause destruction on Earth, nullifying the option for Dean to get the Mark.
4 Dean Said “Yes” To Michael
Just like many times before, Dean's motivations behind allowing Michael the Archangel from the apocalypse alternate world to possess him was rooted in saving his brother (and Jack). Lucifer, after stealing Jack's grace and becoming even more powerful, kidnapped Sam and Jack. Dean's desperate plan to defeat Lucifer as a vessel for Michael led to many deaths. Had Dean allowed the already-injured Michael to die instead of giving him a boost by allowing possession, the hunters from the apocalypse world wouldn't have been slaughtered, almost an entire city of people wouldn't have been turned into monsters, and Garth wouldn't have gotten controlled by Michael, who then attacked Sam and Jack.
3 Sam and Dean Insisted Kevin Translate The Demon Tablet
Kevin was practically lugged around between the Winchesters, angels, and demons after meeting Sam and Dean. The Winchesters convinced Kevin to translate the tablet before they handed him over to the angels. This was where true danger started for Kevin since the angels got killed by a Leviathan, who then took Kevin. Once the Leviathans were defeated, Crowley took Kevin, also to have the tablet translated. If Sam had kept up with his messages after Dean went to Purgatory, he would've known Kevin was in danger and could have intervened, yet chose to cut off everything from his life prior to this.
Aside from the stress and panic from constantly being snatched by various enemies, the act of translating the tablet itself also took a toll on Kevin. In trying to decipher it, Kevin underwent extreme mental anguish, all while cooped up and away from his family. Even worse, later, Kevin's trust in Sam and Dean ultimately got him killed by Gadreel, who was possessing Sam. If he had been working with them and residing in the bunker, Metatron wouldn't have been able to use Kevin as a loyalty test.
2 Sam And Dean Didn't Try To Bring Back Adam From Hell
When Sam, possessed by Lucifer, and Adam, possessed by Michael, got trapped in the cage, Sam was saved by Castiel, yet Adam was left behind. After tenuous interactions with their half-brother and the ongoing theme of family above all else, it's dissatisfying that they didn't even try to save Adam. Additionally, if they had saved Adam, it would've prevented Michael's return in season 15 since he wouldn't have had a vessel to tether to. Michael ultimately betrayed the Winchesters to try and fulfill a need to get God's approval. Also, Adam and Michael bonded over not having families who look out for them, but Michael later revealed that Adam died at the hands of God. If Sam and Dean had tried to save him with the same urgency they save each other, Adam may have lived.
1 Sam Not Destroying The Book Of The Damned
After Dean took on the Mark, Sam's urgency to "cure" him of it and the negative hostility that came with it led to reckless choices, like saving the Book Of The Damned. This powerful tool for witchcraft not only allowed Rowena to be even stronger (and she wasn't exactly a loyal Team Free Will member at that time in season 10 but also led to Charlie's Supernatural death and The Darkness' release into the world. Additionally, the Styne family, who killed Charlie while in pursuit of the book, were brutally killed by Dean, further feeding the Mark's hunger for murder.
Ultimately, Sam and Dean have made a number of selfish or self-serving decisions that resulted in innocents dying or loved ones taking the brunt of the consequences. Often, the brothers' inherent inability to live without each other caused reckless decisions that instigated short-term challenges and multi-faceted story arcs and enemies that lasted across multiple seasons. However, if Supernatural had not prioritized Sam and Dean's relationship, seemingly above all other characters, much of the show's most memorable moments may not have played out in the same way, such as soulless Sam or Dean's demon era.
