I'm not looking for excuses for Euron, but even someone so wholly villainous as Euron has a certain tragic humanity to his character. TWOW spoilers included.
It sounds like Euron was born so psychically powerful so young, but was surrounded by a culture that only recognized literal, physical strength. He had some kind of ability to see some kind of greater picture, to recognize that out there, somewhere, there was the ability to “fly,” but everyone around him is lockstep in this smallminded, delusional ideal of the old way, which is obviously going to doom them.
So he never had any guidance. It’s Bran without Jojen to mentor him, Varamyr without Haggon. We see from Varamyr how totally alienating these kinds of powers are—even though Varamyr uses them to become an utter piece of shit, a lot of it comes from his acute awareness of how much his existence separates him from the society around him. Even with guidance Bran is still warging Hodor to try to pretend to be someone else, he’s still staying in Summer for days at a time to escape his existence as a physically weak skinchanger.
And Euron isn’t just lacking a mentor, he’s lacking anyone who even recognizes what he’s experiencing. He blasphemes against god, killing Harlon, who is in some ways parallel to Euron, also sick, also silently suffering for the unwillingness of the Ironborn to extend empathy past strength. Killing Harlon is like killing himself, Harlon begs him, (to what? stop? continue? he’s not any more sure that he wants to live than Euron is) and immediately afterward, Euron himself is prepared to die, begging god to kill him because even that would be proof that he’s not alone, not the only person who is attuned to these senses. Instead, Euron lives. He’s not struck down—and so he is truly alone. On Pyke, only the gods could relate to the experience Euron is having, and even they won’t answer him.
Eventually Euron leaves, and now, separated from the Ironborn, he lives an empty, macabre replica of their ideals. He rules with absolute authority a ship of mutes, as lockstep with his goals as the Greyjoys were with Balon’s. He’s the perfect pirate but it’s not the plunder he craves—that joy is empty for him. It’s the prayer he that he really wants, the only recognition that the sole, lonely god in the world could ever ask for.
Finally he returns home, killing his brother and stepping into his role, the perfect ideal imitation of what Balon could have been. He recognizes the illusion the Ironborn need. But in private, to his family, he’s still searching for an equal. He wants to invite Victarion into his world—he’s suddenly found a substance that lets average people see the way Euron has been seeing this whole time. But Vic still won’t have it, so Euron decides to forcibly create a brother that understands him. If Vic won’t take the leap voluntarily, Euron will push Aeron—which is cruel, but it is pitiful, too, because he even now he wants someone to know what it's like to see like he does, even now he wants someone to fly with him.
Obviously Euron is still a maniac and a madman but... you can also imagine how lonely he must have always felt, how even his nihilism is tragic—if he thinks himself a god, and the gods are dead, then he knows he is so, so alone.
That said. This excuses nothing and leaves plenty unexplained, too.