The main characters of the 15-season, action-packed, edge-of-seat, heartwrenching show, The entire Winchester family basically grew up with self-sacrifice, even the sacrifice of others. Not only did everyone's favorite brothers live their whole lives making terrifying and upsetting decisions, but other characters also drank from the bad-decision cup as well.
From making deals with demons to actually killing themselves, the core characters of the show made some of the biggest choices ever, that perhaps saved the world but kept burning them. Not only did fans see the characters lose loved ones, either by death or by choice, but they also watched the characters lose themselves sometimes. Losing a soul became one of the main issues in the steadfast decisions to take down one evil being at a time. Talk about some serious self-sacrifice. Dean is known as the character that has made some of the most dangerous and life-altering decisions, but he wasn't the only one with questionable life choices.
Sam Trusting Ruby
This one is a given. Fans know this period in Sam's life all too well, but it truly served as one of his first terrible choices. Not only did Ruby set him up to break the final seal by killing Lilith, but she also led him on the path toward Lucifer.
While Sam did learn from Ruby how to exercise demons properly to save human lives, her manipulation still went a long way with Sam. It was because of her that he dove into the demon blood-drinking lifestyle. Sure, he thought his actions would save humanity, but working with this demon did more harm than good.
Mary's Deal With Azazel (Yellow Eyes)
It was the deal that started it all! Mary Winchester made a deal with Azazel before Sam and Dean were ever born. Yes, this was to save John, but Dean wasn't totally wrong when he blamed his mother for his trainwreck childhood and lifelong personal issues. While Sam and Dean were guilty of making deals to save the ones they loved, perhaps none of that would have been necessary if Mary had let John die and then moved forward.
A clearly difficult proposal, but it would have also saved her from a painful death and an estrangement (while temporary) from her sons in the future. The entire Winchester family's pain could have been avoided had Mary avoided the temptation to resurrect John at the hands of Azazel.
Every Time All The Characters Worked With Lucifer
Every single time. Even when viewers started to see different sides of Lucifer, it still always ended in disaster. While Jack can't really be blamed for trying to connect with his father, he was certainly thrown into the pool of betrayal once it was revealed that Lucifer was going to let Michael destroy the world.
Other times when the main crew chose to work with the devil also ended in more battles they didn't need. From Lucifer's persuasion to possess Castiel in order to fight Amara, all the way to his manipulation in Apocalypse World that he could help get rid of Michael were openings for Lucifer to take advantage of his next opportunity to strike. It became very obvious throughout the entire show that the devil would clearly remain evil.
Sam Saying "Yes" To Lucifer
The surrender of his life led to Lucifer haunting Sam for the rest of the series. Losing his soul, seeing Lucifer when others didn't, losing his mom to Apocalypse World that Lucifer dragged her into, and so on, Sam's agreement with the devil constructed the path that Lucifer was on to ruin everything.
While it was thought that Sam had to say "yes" to him, Lucifer was still going to wind up hurting the future anyway. Besides, the brothers always found a way out of things, as Sam said in Season 14 when Dean was building the Malek box, so there could have been something else later in time to get rid of Lucifer had Sam kept refusing to be his vessel.
Dean Saying "Yes" To Michael
This choice really catapulted the world of the show into disarray. In a desperate attempt to finally kill Lucifer, Dean's acceptance for (alternate universe) Michael to possess him was one of the worst choices he ever made. Michael tricked Dean into thinking he'd have some control, but Michael didn't hesitate in taking advantage of the opportunity to drown him.
Dean, unfortunately, underestimated just how sly Michael was and therefore paid the price for his choice. Michael "left" Dean only to leave room for him to re-possess him and raise his army of vampire-like monsters. Without Jack, Dean probably would not have been saved.
Castiel Letting Lucifer Possess Him
Poor Cas. He really walked into a trap with this decision. As the efforts to defeat Amara grew desperate, Cas allowed Lucifer to share his vessel to fight the darkness. What came out of the whole ordeal though ruined a lot and didn't actually beat Amara in the end.
Lucifer was able to put aside his differences and work with the Winchesters and Chuck to bring down the darkness, but once Amara cast him out of Cas, Lucifer just kept coming back into their lives. If Cas didn't let Lucifer in, the main characters could have possibly avoided future confrontations with the devil.
Dean Allowing Ezekiel (Gadreel) To Possess Sam
Sam really put his brother in place when he accused Dean of allowing the possession because he didn't want to live without his little brother. While it was hard to hear that, he wasn't wrong. Dean's choice to let an angel, that barely anyone knew, resurrect Sam as his vessel caused a huge strain on the brothers' relationship.
Sam was furious at his big brother for his desperation and Dean knew that he couldn't necessarily prove him wrong. Not only was it an emotional problem for the brothers, but this choice also led to a fight against the angel and Metatron.
Dean Doesn't Stop Castiel From Leaving
While this doesn't sound like a life choice, it certainly was on Dean's part because of how he let his anger over his mother's death fill him, and then turn on his best friend, Cas. Once Cas left, Sam and Dean were facing God's worldwide destruction alone, without the help of their right-hand man and angel. Cas had his angelic powers to heal and kill at any point when it was necessary.
While those powers were sometimes limited, it was still better than fighting alone as a human, even though everyone knows the Winchester brothers didn't always need help. Still, Castiel's presence should have been acknowledged by Dean. Losing his friend was not worth the consequences he was on the road to face later in life.
Mary And The Men Of Letters
This could not have gotten any worse because of how bad this choice already was. Mary's struggle to acclimate to the future and bond with her grown sons was justifiable until she thought she was doing justice by working with The Men of Letters.
Ketch figuratively poisoned her mind in welcoming her to their group, and then literally when he brainwashed her. If only Mary hadn't run off to work with this team of traitors, Sam and Dean wouldn't have had to risk their lives to save her and defeat The Men of Letters' emotionless army.
Dean Accepting The Mark Of Cain
From "Deanmon" to the murderous rage, to almost killing Sam, Charlie's death, and unleashing the Darkness, Dean's choice to bear The Mark of Cain caused such a tumultuous domino effect on all of the main characters. He was warned by Cain that The Mark was a huge burden, but Dean didn't give himself much time to contemplate it in his efforts to kill Abaddon. While Dean managed to beat Abaddon, he couldn't fight off the evil that raged inside him from The Mark's power.
Dean's choice led to the loss of a beloved character (Charlie) and to his eventual demonic resurrection after Metatron killed him. Demon Dean was the ultimate "lean, mean, Dean" killing-machine. Finally, the Darkness was the final result of The Mark, leading to even more issues for all the main characters to deal with. Bearing The Mark was possibly Dean's most scarring (no pun intended) choice.