The Game of Thrones season premiere featured a brief line from Euron Greyjoy that alludes to one of his more heinous habits from the books: cutting out the tongues of his Silence crew members. The show's version of the Ironborn reaver has differed significantly from his counterpart in George R.R. Martin's A Song of Ice and Fire saga, but "Winterfell" confirmed the two characters share one significant and cruel similarity.
At the end of season 7, Euron took his ships and sailed for Essos to acquire a significant prize for his queen: a deadly group of mercenaries known as the Golden Company to aid her when the surviving army of the war reaches Kings Landing. Regardless of whether or not Euron believes the Night King to be an actual threat, it remains a low priority compared to his desire to court Cersei's favor and increase his standing on the global stage. It's an entirely self-serving move, characteristic of the brutal pirate introduced in season 6 who murdered his brother in his very first scene, and has since become a dynamic member of the show's deep bench of depraved villains.
While visiting his niece in captivity on Silence, Euron reveals to Yara that the reason he keeps coming to talk to her is that he's on a ship "full of mutes," and it gets lonely at sea. He's referring to his practice of cutting out the tongues of any sailor that serves on Silence. Euron's Greyjoy has a reputation in the books as a ruthless, amoral sailor, supported by a number of myths surrounding his brutality. Most of those myths turn out to be true, and forcibly muting his crew (and his pregnant mistress) is one of them.
Reasons for this practice range from paranoia to megalomania to sadism, and none are mutually exclusive. In the books, Euron - like Walder Frey, Ramsay Bolton and Joffrey Baratheon before him - establishes his villainy by gleefully engaging in behavior that flies in the face of what little shared cultural morality Westeros maintains. He kinslays without compunction, is loyal only unto his own ambition, and is the only major villain that openly lives outside the law. The series used those deeds to outline the character played by Pilou Asbæk, but also infuses the character with a more lighthearted air of wickedness.
In the show, Euron Greyjoy feels like a psychotic Jack Sparrow, who is willing to light the world on fire because it makes for such a good show. He's power-hungry to be sure, but his ruthlessness comes tempered with more humor and mischief than could ever be attributed to his book counterpart. Still, the fact that he mutilates his own crew fits with both versions of the character, and that's a credit to the show knowing its limits when it comes to adapting Martin's work.
Distilling Martin's dense storytelling down to about 80 hours of television ensured some of the author's work wouldn't be adapted perfectly or included at all. Certain character interpretations have been criticized by book fans, and the version of Euron Greyjoy that makes bad dwarf jokes (and spawned countless Euron Greyjoy as your mother's new boyfriend memes) certainly seemed poised to rile. But it's precisely because Euron Greyjoy has such a depth of character in the books that it was a smart move for Benioff and Weiss to embrace what's almost a comedic interpretation of the character in the novels.
When Euron Greyjoy was introduced, the show had just shy of 20 episodes to go, and there simply wasn't time to build Euron up as someone who could rival Ramsay Bolton in depravity and audience engagement. Keeping the spotlight on Cersei and the Night King as the series drew itself to a close made sure the integrity of the show's narrative remained intact and undiluted by an ambitious introduction of a new character that was never going to be able to evoke the original successfully in the time remaining. That said, dropping in the fact that he mutilates his own crew says a lot about Euron in a very short amount of time.