In Season 6, Episode 5 of the show, after his Drowned God ceremony, Euron mentions wanting
to find and murder Theon and Yara.
Why does he want to do this?
In Season 6, Episode 5 of the show, after his Drowned God ceremony, Euron mentions wanting
to find and murder Theon and Yara.
Why does he want to do this?
While there may well be deeper reasons, at the very least, refusing to attend your new king's coronation is a pretty clear sign of disloyalty, disloyalty which could (and, in fact did, although it's unclear whether anyone there knew about it) lead to Theon and Yara taking their supporters into actual treasonous action. In a world like GoT, murdering potential leadership rivals seems like standard policy.
Would it have been avoided if they'd openly bent the knee? I doubt we'll ever know.
In rises of power, many times the opposition is eliminated afterward in order to prevent them from uprising or resisting your rule. By killing Theon and Yara, he would essentially be removing any contenders to his crown and, in his eyes, preventing any possibility of regicide.
Assassination of Renly Baratheon after the failed meeting between Stannis and Renly concerning the Iron Throne.
Jon Arryn, assassinated by Lysa Arryn and Littlefinger to ensure Lysa's control over the vale.
Robert Baratheon died from boar hunt after becoming drunk on wine given by Lancel Lannister at Cersei's command.
Eddard Stark executed on Joffrey's orders after being instructed that he should rule instead of Joffrey by Robert on his deathbed.
Attempted assassination of Daenerys Targaryen by merchant ordered by Robert Baratheon.
Red Wedding, assassination of Robb Stark, Talisa Stark, and Catelyn Stark by Walder Frey.
Assassination of Joffrey Baratheon by Littlefinger and Olenna Tyrell.
Assassination of Lysa Arryn by Littlefinger.
Myrcella Baratheon poisoned by Ellaria Sand of the Sand Snakes.
Doran Martell assassinated by Ellaria Sand Snakes.
Trystane Martell assassinated by Obara of the Sand Snakes.
Roose Bolton assassinated by Ramsay Bolton.
Walda Bolton assassinated by Ramsay Bolton.
Balon Greyjoy assassinated by Euron Greyjoy.
Dothraki leaders assassinated by Daenerys Targaryen.
By rules of succession common throughout Westeros (and Earth), Euron is either second or third in line to the Salt throne behind Theon and (possibly) Yara. See this question for details.
The Iron Islands have their own traditions (It seems to be based on a form of popular support), but they are clearly influenced by others as both Theon and Yara were seriously considered as successors.
By killing Theon and Yara, Euron strengthens his claim to the throne by following both traditions - he has both the popular support and the usual line of succession.
Incidentally, this is also why he wants to marry Daenerys. It's possible to take the Iron throne without any claim at all (see Robert Baratheon), but it's easier to convince the people to follow you if your claim can be backed up by traditions of succession - see Joffrey & Tommen Baratheon and the hiding of their true lineage)