Most of the characters in the Marvel Cinematic Universe are human but with slightly altered genes. What that means is that they are prone to make mental errors in judgment. However, even those from other planets and worlds are prone to the same issues. As strong as Thor is and being labeled a God, did he make all the right choices while defending Asgard?
Captain America is viewed as the most righteous member of the MCU and even he made mistakes. But who made the biggest and why? Did their errors in judgment result in a loss of life like Gamora's with Thanos? No matter how viewers feel about their heroes, they've all made some sort of mental errors.
Alex Wilder (Ambition)
Alex was the only one of The Runaways not blessed with some form of power or weapon. He was labeled the leader of the team thanks in part to his brain. But there was still something lingering inside of him just waiting to get out. What he wanted was to be better than them all.
At the end of Season 3, Xavin saw the future and what would become of Alex. His error in judgment would be his desire to allow his greed and jealousy to consume him. He betrayed his family.
Luke Cage (Taking Over Harlem)
Luke Cage has a deep love for Harlem. He fought to protect the city and the people in it. Crime boss after crime boss he defeated to stop the madness in his city, and in a moment of desperation, thinking he was doing the right thing, Cage may have done more harm than good.
At the end of Season 2, Luke Cage was seen at the club overlooking the patrons the same way Mariah and Cottonmouth did when they ran the city. He even turned Misty and Claire away, cementing the stance he had taken. Even with a bulletproof body, Cage could have left well enough alone. He made a terrible judgment call as the head of Harlem.
Captain America (Ego)
Captain America has made a few questionable calls during his time in the MCU. But the main one has to be him thinking and believing that he was built to lead The Avengers. While situations may have placed him in a position to give orders, no one on the team really needed his guidance.
Cap has always had an ego, but leading The Avengers is also one of the reasons why they broke up. Cap against Tony Stark was more personal than anything else. Stark wanted the mantle, but Cap felt it was his to keep. His worst judgment call was believing he could handle it all on his own.
Spider-Man (Self-Pity)
Marvel fans still must remember that Spider-Man is just a high school kid. Saving old ladies and bodega store owners from robbers is one thing, but Parker was dealing with adult issues ever since Tony Stark came into his life. In Spider-Man: Far From Home, Parker was still dealing with the death of his mentor, Tony.
In a moment of self-pity he gave Stark's last gift to him (E.D.I.T.H.) to Quentin Black AKA Mysterio, thinking that he was the one to carry the next hero torch. In the process, Mysterio used E.D.I.T.H. to cause havoc. It's hard to fault the young hero, but it was a decision that he wishes he never made.
Daredevil (Elektra)
No matter how much good Matt Murdock did in the courtroom, his other job as Daredevil forced him to make decisions that weren't considered great judgment calls. In Season 2 of Daredevil, Matt found himself reunited with his ex-girlfriend Elektra. A romance blossomed but then it all went sour.
In The Defenders, Elektra returned, and while she was hell-bent on killing the team, it was Daredevil who tried to reason with her. Instead of fleeing with his team in the final battle, Daredevil decided to stay with Elektra. A choice that almost cost him his life.
Bruce Banner (Building Ultron)
All the blame can't be placed on Bruce Banner. While looking for a way to create the ultimate Artificial Intelligence, Banner and Stark create Ultron. The idea was good, but the end results caused a massive war and changed The Avengers forever.
As smart as both Banner and Stark are, they should have known that something could have gone sideways. While Vision was born from their error, Ultron caused damage that took years to be fixed.
Gamora (Caring For Thanos)
For a character to hate someone as much as Gamora says she does Thanos, her terrible judgment may have cost her her life. In Infinity War, Gamora and Thanos went head-to-head, with Gamora seemingly coming out the victor. But it was Thanos using the Reality Stone and playing games with her head.
Gamora in tears was a bad judgment call on her part, but it was pure emotion. Despite her ill feelings towards him, there was still a piece of her that cared for him. Thanos would later use that against her.
Nick Fury (Waiting To Call Captian Marvel)
Captain Marvel was the first hero that Nick Fury ran into before his journey as the head of SHIELD. His time with Captain Marvel gave him the idea for the Avengers, and when Thanos made his appearance and started wreaking havoc, why did he wait so long to get in contact with her?
As powerful as Thanos was, she could have handled him herself. What was the hesitation about? And not just Thanos, where was she from the start? Fury had the greatest poker hand and decided to fold every time when he should have gone all in.
Tony Stark (The Sokovia Accords)
Due to issues that may or may have not been avoided, the government approached Tony Stark with an idea called the Sokovia Accords. What the government wanted was to keep their eyes on the heroes and hold them accountable for their actions. Tony agreed, but his actions caused conflict within The Avengers.
The outcome was the dissolved relationship between him and Captain America and a split down the middle of the team that cultivated in a war of heroes then a battle between Iron Man, Cap, and Winter Soldier. Tony could not see the bigger picture until it was too late.
Star-Lord (His Emotions)
There was a moment in Infinity War where the good guys actually had Thanos beat. Mantis had him under a spell and it was all about to be over as the team was set to grab the Gauntlet off his hand. But Star-Lord, still dealing with the grief over Gamora, just couldn't contain his emotions.
While getting in Thanos' face and asking where Gamora was, he lost it and punched Thanos. That hit allowed him to break free of Mantis' hold on and well, the rest is history.