Joffrey was basically a bully to the butcher’s boy, so Arya and Nymeria put him down. Sansa lied about it later on, Her lies resulted in Lady being killed. If she told the truth, Lady and Nymeria might’ve gotten more time.
(Season 1, Episode 2: “The Kingsroad”)
19. Cersei ordering Lady to die.
Cersei ordered Lady to die once no one was going to get punished for Joffrey’s attack. If Sansa told the truth, perhaps this wouldn't have happened.
(Season 1, Episode 2: “The Kingsroad”)
18. Ned keeping Cersei’s incest a secret from Robert.
Ned should’ve told Robert about Cersei and Jaime’s incest once he learned of it. Because he kept it a secret, it ultimately resulted in his death and Robert’s, and everyone else’s since it set the war in motion.
(Season 1, Episode 7: “You Win or You Die”)
17. Theon betraying Robb.
Theon betrayed Robb for his father. Not only was Balon never thankful, he didn’t even try to rescue him later when he got captured. Theon betraying Robb resulted in the Starks losing Winterfell until Season 6. Karma bit him in the ass later though, he redeemed himself by helping Sansa escape, and denouncing his claim in favor of his sister Yara.
(Season 2, Episode 2: “The Night Lands”)
16. Catelyn setting Jaime free.
Catelyn was a frustrating character, but one of the worst things she did was set Jaime free in exchange for her daughters. It was a emotional move to let such a highly prized prisoner go, even Robb realized that.