You're not wrong, but no one cares that he killed Balon and got away with it. Iron islanders are notorious killers.
The first thing Balon asks Theon is whether he killed someone to get his clothing.
Whether it rings true in the books, the characters in this show kill each other and get away with it because they are seen as strong enough to get away with it. That may ring thematically false, but there are always bullies throughout history.
They're not notorious kingslayers. They wouldn't be around if they just killed their leaders all the time...
They kinslay and get away with it because it's convenient to the writers and allows them to set up situations that wouldn't happen otherwise. That's all. It's not thematically false, it's just false, as it throws the rules of the world and established mythology to the wind for the sake of cheap plot contrivance.
Ramsay, Euron and the Sand Snakes are
NOT "strong enough." That's the point. That's why it's bad writing. They get away with inexplicable actions with very, very, very bad justifications. In the books, Euron wins over the Iron Islanders because he's not only charismatic but he brought back a lot of loot and more, in the show he wins over the islanders by repeating Yara's proclamation and inserting a few dick jokes. The Sand Snakes get away with it because apparently Doran's people hate him, despite there never being a single hint of this until it was convenient to write him out of the show in one of the most Poochy-esque ways imaginable.
If the show was written with reason and logic in mind, Ramsay, Euron and the Sand Snakes would not succeed against their relatives so easily. But the show writers did not want to formulate actual situations where they could, so they went with the low hanging fruit and reused kinslaying several times, cheapening that aspect of the world.
Tyrion is accused of killing his king, and then chooses to kill his father. Neither of those things are seen as worse in the show because of their family relations. They're seen as horrible because of the powerful positions these men inhabited.
Again, no one in the show says the word kinslaying. What are you talking about?
Tywin says he would've killed Tyrion if he wasn't a Lannister, due to the taboo against kinslaying. Victarion relents on killing Euron in the books for that very same reason, so your excuse is nonsense. Kinslaying is as taboo as incest or violating guest right, the only difference is that in the show only the latter two are shown to have ramifications.