Euron seems more ambitious than sadistic. He wants to take Jamie's place at Cersei's side.
He wants to be a king, in some manner, so first he kills his brother in the Iron Islands and then he smashes up Theon and Yara. Now, he's basically just kissing Cersei's ass until he can marry her.
He enjoyed the celebration as he pulled Ellaria and Yara through King's Landing. - so that right there shows you what he truly wants. In fact, I think he said "This is great!" or something like that while riding the horse to the palace. He loves feeling like a celebrity.
He just wants to be a somebody and will do whatever he has to do become one. He doesn't come off as a sick twisted torturer like Joffrey or Ramsay.
I mean, that could change, but so far all he's done "bad" is kill his brother and taken his Niece prisoner - but she tried to claim the Iron Islands too, so that's more politically motivated than anything. Everything else he's done has been mostly by request. In fact, his naval battles have been pretty impressive to be honest.
Now, if they start off the next episode with him torturing Yara just for giggles, then things change and he becomes that sicko the previous seasons had.
