In the Star Wars universe, the decisions made had insurmountable consequences, whether big or small. In particular, the bad decisions had ricochet effects across the whole of the galaxy, and the characters in the sequel trilogy fell victim to these circumstances, some more than others.
Characters such as Kylo Ren had a tendency to commit grave mistakes in his leniency to the dark side, whilst the more good-willed characters, like Rey and Leia, made bad decisions out of impulse or their commitment to duty that was usually clouded in the light. Either way, each main character made at least one bad decision along the way.
Rose: Trusting DJ
When Rose was first introduced in The Last Jedi, it was clear that she was not to be messed with. Despite her overall sweet nature, she appeared highly skeptical and even distrusted Finn initially. When she and Finn first encountered the code-breaker DJ on Canto Bight, Rose was immediately wary and didn't trust them at all.
Yet, when DJ appeared to save them (and himself) and helped them break onto the First Order's ship, she grew to trust him, which ultimately put her and Finn in great danger as DJ sold them out. Whilst it was Finn's mistake too, Rose tended to be a level-headed character who didn't make many bad decisions, except for this oversight.
Snoke: Putting Too Much Faith In Kylo
However underutilized he may have been, Snoke was a menacing villain in the sequel trilogy. He was merciless in his manipulation of Kylo and had brutal interrogation tactics. However, he was also blinded by his own ego - a grave mistake that had fatal consequences.
Snoke put too much faith in Kylo's link to the dark side, believing that he had brainwashed him enough to be persistently loyal to him and their cause. Had Snoke chosen to be warier of Kylo and his lineage, he may not have died at his hand, yet he singled Kylo out from a young age and believed in his power - a decision that got him killed.
Hux: Losing Focus Of The First Order's Mission
It was both shocking and highly amusing when Hux revealed himself to be the mole working for the Resistance in The Rise of Skywalker. His commitment to the First Order's cause seemed to get shrouded in his hatred of Kylo, and his decision to help the Resistance because of this was fatalistic.
Had Hux had more pure intentions in his efforts to help the Resistance, he may have not acted as irrationally. As it was, he was too focused on the demise of Kylo Ren that he put himself in the firing line (literally) with his First Order superiors. It was a string of bad decisions from a usually sensible and intelligent man, and Hux paid the ultimate price for them.
Han / Leia: Sending Ben To Train With Luke
At the heart of the sequels was the pain that permeated through the original trio at the loss of Ben Solo to the dark side. However as much as Han, Luke and Leia may have been wasted, their actions prior to the trilogy set the events in motion, and Han and Leia's decision to send their son to train with Luke had a cataclysmic effect.
Both parents expressed tremendous guilt for the choices they made, and they suffered greatly for the decision - not only did they lose their son, but he became part of the enemy faction fighting against them. Han and Leia realized that in their want to protect Ben from the dark side, they had essentially banished him to suffer alone through the Jedi Order's strict code, and was thus the perfect prey for Snoke, but their realization came too late.
Luke: Nearly Killing Ben Solo
Like Han and Leia, Luke expressed much sorrow for his part in the downfall of Ben Solo, most notably his distrust towards him whilst Ben was in his care. Luke's cynicism towards the Jedi's ways was the result of how he treated his nephew, as he became too blinded by protecting the light to see that Ben needed help.
Instead of offering guidance, Luke acted on impulse and nearly killed Ben in his wayward intent to stop the darkness rising in him, and the galaxy. Although Luke repented for his actions, the consequences of his split-second decision were irrevocable and the catalyst that Snoke needed to bring Ben fully into the dark.
Poe: Disobeying iral Holdo
Poe was a particularly impulsive and hot-headed character who didn't respond well to authority. Whilst these traits sometimes paid off, his decision to defy iral Holdo in The Last Jedi jeopardized the Resistance massively, and created unnecessary chaos as they tried to escape from the First Order's clutches.
Poe was too caught up in his own ideas of how to fight the First Order to see that Holdo was actually doing the right thing by the Resistance and Leia. Rather than respect Holdo's plan, he attempted to overthrow her authority, which resulted in him getting stunned by Leia, and the Resistance wasting precious time with the inner politics of their leadership. However good his intentions, it was a juvenile decision that he thankfully learned from.
Finn: Trying To Sacrifice Himself
Finn's worst decision was simultaneously his most heroic. His sacrifice attempt at the end of The Last Jedi was just one of many ways that Finn's story could have turned out differently and had Rose not stopped him, his death would have had a massive impact, particularly on his friends.
His decision to sacrifice himself was a noble act indeed but ultimately resulted in Rose getting injured to save him, with the Resistance managing to escape from the First Order anyway. His choice was also bound up in his hatred towards the enemy, which made him act irrationally, and was therefore the worst decision he made, however honorable it may have been.
Kylo: Staying With The First Order
Kylo Ren committed many shameful acts throughout the sequel trilogy, with the murder of Han Solo coming in at number one. However, considering that Kylo had been manipulated a great deal of his life by Snoke, it can be said that he didn't necessarily decide to do that for himself, as he was coerced into darkness by his former master.
Therefore, his decision to stay with the First Order after killing Snoke and finally becoming free of his manipulation was one of the worst decisions Kylo actually made for himself. After saving Rey (and himself, presumably) he tried to convince her to his cause, rather than to return back to the light with repentance and forgive his family. For a man who had spent much of his life as a puppet to different masters, it was a poor choice to remain in their shadows.
Rey: Exiling Herself
Throughout the sequel trilogy, it was repeatedly proved that the Resistance needed unity above all else in order to bring the First Order down, yet Rey fled from her friends in The Rise of Skywalker for a Luke-inspired exile on Ahch-to. After the death of Leia and the near-death of Kylo, Rey decided the fate of her friends would be better if she weren't around.
By the trilogy's third film, Rey had weathered her fair share of pain and it was understandable that she would want to be free of the expectations placed upon her, but her decision was born out of anger and pain, rather than a yearning for peace. Thankfully, Luke's force ghost reminded her of her purpose, but the decision to abandon the Resistance in the first place was still a bad one.
