He doesn't misunderstand what you think the character should be. He was given a different task, which apparently was to do something completely different than what we're used to in the most popular Luthor versions.
His character is goofy and awkward, which isn't necessarily wrong given that's exactly how many of these super smart people are. He is also conflicted and tormented by his inner demons and that can easily be seen in the weird way he articulates himself. You can feel his lunacy in his voice and mannerisms. He is a young genius that's completely insane and emotionally scarred, and that actually justifies his actions in the movie. He's not a stable person and that's why he gets screwed by his own genius intellect.
The way the character was portrayed is nothing more than a creative choice. He certainly doesn't have that typical powerful bad guy look that is so popular and so cliche, since Luthor doesn't really look/sound/acts that much different than many other bad guys. But at the end of the day, he still hates superman, he still comes up with crazy plans to beat him, he's still a genius and to me that´s Lex Luthor.
Go read Superman comics and you will find several Luthor versions through out the history. Some of them not that far from what we saw in the movie.
