Moral Event Horizon / A Song of Ice and Fire

• Tywin Lannister crosses this before we even properly meet him. We hear in the backstory how he order the sacking of King's Landing which killed hundreds, but it gets worse when Tyrion tells the story of his first wife. He secretly married whom he thought was a crofter's daughter whom he and his brother Jaime had rescued from being raped. When Tywin found out he told his son that she was a whore hired by Jaime to give Tyrion his first lay. To prove it, he had her gang-raped by a barracks full of Lannister guardsmen, even paying her a silver piece for each one, and then forced Tyrion to go last. It's made even worse when we later find out it was a lie. She really was a crofter's daughter who fell in love with Tyrion. When he learned this, Tyrion flew into a murderous rage and killed his father.
• Joffrey Baratheon, who decided to execute Ned Stark, the Decoy Protagonist and viewpoint character of A Game of Thrones, despite promising his girlfriend and Ned's daughter, Sansa, that he would be merciful and allow Ned to join the Night's Watch. He then proceeded to become The Caligula at age 13. It could also be argued that Joffrey's MEH came much earlier, when his lie caused the deaths of Sansa's dire wolf Lady and Arya's friend Mycah by demanding their executions over a minor injury that neither was responsible for.
• Gregor Clegane's MEH came before the start of the series, when he murdered a baby by dashing its head on a wall, then raped the mother with the boy's brains still on his hands. And this is even putting aside the fact that when he was a child, he found his little brother playing with one of his toys... so he held his face in a burning brazier until half the flesh melted off.
• One of the most chilling MEH's of the novels comes when we find out what happened to Ramsay Bolton's wife: He locked her in a tower with nothing to eat. They found her with no fingers and blood around her mouth.
• Once upon a time, Walder Frey and his brood were obnoxious hillbillies that the rest of Westeros had to tolerate because they held a major crossing. Then the majority of the named family members decided to massacre Robb and Catelyn Stark and the Northern army at the Red Wedding. Now readers cheer when random Frey children and grandchildren end up killed and used as the filling for delicious pies. It was especially the method (betraying Sacred Hospitality) by which the Freys did it that attracted such hatred.
• Sociopath Roose Bolton first bent the knee to Robb Stark, then crossed the MEH when he plotted with Tywin Lanister to convince Walder Frey to host the Red Wedding and served as the Starscream by personally murdering Stark at his own wedding.
• Also played with early in Game of Thrones when Jaime Lannister pushes the young Brandon Stark out of a window for witnessing him and his twin sister, the queen, having sex. This seems at the time to be a definite moral event horizon, but Jaime later becomes a gradually more sympathetic character after he undergoes extreme suffering over the course of the story, which prompts a definite change in his moral character for the better.
• Theon Greyjoy proves just how powerful a MEH can be for a Jerkass Woobie. He allows Reek to kill two Small Folk children and their mother along with a handful of other people to cover up the escape of Bran and Rickon, and spends the rest of the series paying the horrific consequences.
• Rorge once found a young boy, took him in, filed his teeth and made him fight dogs until he went mad. You now know that boy as Biter.
• Viserys Targaryen threatened to cut Daenerys's baby out of her womb because her husband took too long to keep a promise.
• Randyll Tarly crossed by either threatening to arrange a Hunting "Accident" for his own son unless he joins the Night's Watch, or even earlier in the backstory by chaining said son to a wall for three days straight because he wanted to become a maester.
• According to Littlefinger, Cersei Lannister crossed it at Casterly Rock when she had bastard twins of her husband killed and sold their mother to slavery years before the main series.