From season one, Game of Thrones established itself as a series unafraid to kill off major characters with the shocking death of Ned Stark. Throughout its run, the show was known for including major deaths in every season, and at times, in almost every episode, with many of them truly shocking or heartbreaking (like Hodor's final name reveal). However, not every death was heart-wrenching or a tragic shock.
Sometimes, characters essentially brought it on themselves with terrible decision-making. Not those who made bad, but understandable, choices - like Shae putting herself first, which was an utter betrayal, but would have seemed like a smart choice at the time. No, these were the decisions that had fans wondering from the start how the soon-to-die character could have ever thought their deadly choices were a good idea.
Viserys - Disrespecting A Dothraki Khal
Viserys Targaryen was a singularly unlikeable character, and fans soon began to wonder how he managed to survive life on the run for so long. Unlike his sister, he refused to adjust to life with the Dothraki, or to show them respect. Instead, he made rude demands from Drogo, drunkenly threatening his unborn child and telling Drogo he would steal his wife 'back' (as Targaryens traditionally intermarried, and Viserys clearly felt some level of ownership). Unsurprisingly, Drogo brutally killed him, and it's astounding that, even drunk, he would think it a wise decision to insult and threaten a Dothraki Khal.
Robb Stark - Reneging On A Deal
Fans sympathize with Robb Stark, because he reneged on a tough deal in order to marry for love. A fan-favorite, everyone was rooting for the young and honorable Stark son, but his death - and the death of his wife, mother, unborn child, and all his men, was entirely his own fault. He made a deal with the Freys, knowing their importance, reputation, and how highly they held their honor. Choosing to let them down, and then to return to them and suggest that Edmure, not the King (or presumed future King, if the war was won) was an acceptable replacement, was never going to end well.
Doreah - Betrayed Dany
Betraying powerful rulers is rarely a smart decision, but it can turn out well, if played carefully. However, Doreah did not make a clever enough choice when she betrayed Dany to align herself with Xaro Xhoan Daxos, one of many characters trying to steal her dragons.
This was a terrible decision on many levels, and not least because she has seen firsthand how Daenerys deals with her enemies, and how powerful she was becoming. It's arguable that Doreah was making this choice for love, but her decision to turn against Daenerys was always going to end up with her death.
Kraznys mo Nakloz - Tried To Take Dany's Dragons
While Daenerys was still across the sea, and her dragons were still relatively small, several people tried to steal them, bargain for them, or buy them, including Kraznys mo Nakloz and earlier, Pyat Pree. Everyone who made this decision to try and take her dragons away ended up dead, whether burned to death by the dragons, or (in Xaro Xhoan Daxos' case, locked in his own vault to starve) - and no wonder. It shouldn't have taken too much intelligence to realize that someone calling themselves the 'mother' of dragons was not viewing them as belongings to be bought and sold, and that getting on the wrong side of a dragon mother was not a smart move.
The Red Viper - Grandstanding Instead Of Killing
The death of Oberyn Martell shocked fans - and given that he was both a fan-favorite himself, and the Champion of ultimate fan-favorite Tyrion, was hugely disappointing. It was also entirely unnecessary. In combat, he had the upper hand on the Mountain (no mean feat), and had every opportunity to kill him and emerge victorious and alive. However, the Red Viper was overconfident, and chose to grandstand and monologue instead of getting the job done as soon as possible. As a result, he gave the Mountain an opening, and had his skull crushed.
Jaime Lannister - Returning To Cersei
Jaime Lannister's end was one of the most disappointing in the series, as fans watched him grow and change over multiple seasons, eventually falling for Brienne and joining the Starks in the battle to save Westeros. It was an incredible redemption arc, and then he threw it all away by running back to Cersei in the final episodes, only to end up crushed by the collapsing buildings of King's Landing.
Had he simply stayed up North, he could have had a happy ending with Brienne, but instead, he chose to run back into a city as it collapsed, for a woman whose own decisions led to the death of all their children, and who he had a toxic and incestuous relationship with.
Smalljon Umber - Betrayed the Starks
Betrayal is a common theme in the series, and Smalljon Umber is one of the biggest traitors to the Starks. He is the man, sworn vassal to the Starks, who captures Osha and Rickon, kills Shaggydog, and turns them over to the Boltons. This is a terrible decision because it is a terrible betrayal, but more than that, it does eventually get him killed. As an ally of the Boltons, Umber fights in the Battle of the Bastards, and ends up being killed by Tormund. Given that so many died in the battle, it is possible that Smalljon would still have met his end this way had he not made the choice to turn over the youngest Stark. However, Jon initially did not want to fight at all, and had Ramsey not held and threatened Rickon, the battle may never have happened.
Dickon Tarly - Wouldn't Bend The Knee
Arguably, this was an honorable decision, but still a fairly stupid one. Dickon Tarly briefly appears in season 6 and 7, and is driven almost entirely by his loyalty to his family. As such, he decides first to betray the Tyrells and march with the Lannisters, and then when Daenerys ambushes the Lannister forces, he refuses to bend the knee to her. Unsurprisingly, he goes up in flames for this decision. It's a particularly frustrating choice because Dickon has just shown that his loyalties can be flexible, as he turned against the Tyrells, and he was told directly he would die if he didn't submit.
Janos Slynt - Betrayed Ned Stark
Of the many people who betray the Starks in Game of Thrones and later die for it, Janos Slynt's death is possibly the most satisfying. The Commander of the City Watch, part of his job is to protect the Hand of the King, and yet he betrays Ned to Cersei and kills his men. Tyrion, when he is made Hand, sends Slynt to the Wall for treason for this act - and Jon then beheads him. This wasn't specifically because of his betrayal of Eddard, but connects directly to it, and is the result of a series of bad decisions on Slynt's part.
Qyburn - Created The Mountain That Killed Him
Poetic justice also comes for Qyburn, when he is finally murdered by the Mountain - a fitting end as a result of the decision to keep the Mountain alive by turning him into a twisted, half-dead version of his original self. The Mountain spends several seasons as this zombified creature, at the beck and call of Qyburn and Cersei, so it was incredibly satisfying to see the Mountain turn on the man who subjected him to horrifying experiments and then used him as protector for so long. Had Qyburn simply allowed the man to die naturally, he may have escaped Kings Landing, in the end.