IN THE SEASON FOUR finale of HBO’s Game of Thrones, Tyrion Lannister goes on a cold-blooded killing rampage. After escaping the dungeons with the help of his brother Jaime, Tyrion murders his former lover, Shae, and his father, Tywin Lannister. Tyrion’s revenge is the culmination of years of development, an apex of the tragedy that is his character. But what is most remarkable about this moment in the TV series is that the show gets it wrong.

It’s not only that events unfold with very different psychological beats in George R. R. Martin’s A Storm of Swords, the third book in the Song of Ice and Fire series. While HBO’s bungling of Tyrion’s revenge arc is the most glaring of its errors where Tyrion is concerned, this failure is a culmination of mistakes made since the show began. And in betraying the character of Tyrion, the show has deprived viewers of the most compelling character in Martin’s books. This is especially egregious because while the structure and strategy of A Song of Ice and Fire resist having a center — there are so many plots, so many characters — Tyrion nonetheless emerges as the heart of the series.

It’s important to note here that there is nothing negative to be said about Peter Dinklage, a stellar actor who delivers Tyrion’s acerbic wit to perfection. He should get extra credit for his adeptness with scenes so poorly written they veer toward the embarrassing, as when Tyrion is pleading with Shae to believe he thinks she is “the prettiest.” (So much for the intrigues of King’s Landing.)

The blame for HBO’s failed Tyrion rests squarely with the showrunners and writers, who seem to have missed the core element of his character in the books: Tyrion is a tragic character, a cross between Cyrano de Bergerac and Caliban. He is hideously ugly, and following the wars in A Clash of Kings is rendered even uglier with facial damage. Unloved throughout his life — hated by his father, the odd one out of his siblings’ very close relationship, loathed by the bigoted public for being an ugly dwarf, his most loyal man a paid mercenary — Tyrion is intensely isolated, and of course we are charmed by his ability to make a joke of it. But the pain is nonetheless the center of his being. It is this balance of tragedy and a survivor’s wit that draw the reader so close to Tyrion; we are enfolded in his loneliness and applaud his resilience.

When Shae comes into Tyrion’s life, he lets himself believe she loves him. And here’s the important part: she doesn’t. Not one to shy away from the uglier side of human nature, Martin creates in the character of Shae a hollow, duplicitous woman whose only desire is Tyrion’s wealth. The moment it becomes dangerous to be associated with him, she switches sides without a qualm and humiliates him before the courtroom, with detailed accounts of calling him “my Giant of a Lannister.” Tyrion, diminutive and hideous, becomes an object of mirth, on top of the contempt everyone already feels for him. His universal rejection, which began when his mother died giving birth to him, reaches a pinnacle in this scene.

Maybe HBO had good intentions when they upended the character of Shae and made her warm and loving towards Tyrion. It’s possible that the idea of a shallow prostitute was not progressive enough for TV, and deepening a character beyond a stereotype is always a noble goal for a writer. The problem, however, with making Shae love Tyrion, is that it trivializes his story. He is no longer a tragic character — he is loved. In the context of the show this is believable, since Peter Dinklage is a handsome man, and his subsequent scarring in battle could be considered sexy. (It’s absurd that Sansa is so repulsed by the idea of sex with him, in this context — sure, he’s not the man of her dreams, but after the traumas she’s experienced, you would think her fantasies could realign to accommodate a difference in height.) Instead of an isolated, ugly Tyrion, the show gives us an attractive Tyrion with a devoted and beautiful mistress who only turns on him after he abuses her verbally in the most monstrous way. (Yes, it’s deliberately “for her own good” so she will leave him, but only if you imagine she has the intelligence of a ten-year-old.) His revenge, when it comes, already looks different in that light.

One could be forgiven for thinking that the courtroom scene is Tyrion’s lowest point — not only is he believed a murderer, he is a public joke. But in the books, there is actually one final place to drop to, the bottom of the well, and it is this that defines the scenes of murder that follow. In A Storm of Swords, when Jaime frees Tyrion, he reveals that the “whore” Tyrion had married had not been a whore after all, but a willing girl. That when she was gang-raped by Tywin’s guards and banished, it was just that — gang-rape, not a transaction of sex for money as Tyrion had thought. Through the years, Jaime had let Tyrion believe that a prostitute had been paid to seduce him into marriage, when in fact it had been an innocent love affair after all.

In that moment, Tyrion’s entire conception of his life is shattered. It turns out he was loved, once, and that the woman who loved him was destroyed by his father. By the time he reaches his father’s room and discovers Shae in bed there, events unfold in the only way they can. It is a horrifying moment, but it also feels like the inevitable outcome of years of suffering.

On the show? Jaime frees him, they hug, and they go their separate ways. Then Tyrion discovers the woman he has verbally abused in his father’s bed, and finishes her off. Sure, she goes for a knife, so the act can be spun as self-defense, but he pries the knife away very early in the struggle; at that point, he has a number of choices that do not have to involve killing her. And he does. He kills a woman who loved him, whom he had treated terribly. This handsome Tyrion is not one whose story I want to pursue.

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Ilana Teitelbaum’s debut novel, an epic fantasy called Last Song Before Night, will be published by Tor/Macmillan in 2015.