I mean, never attribute to malice what can be attributed to stupidity.
As I understand it, there is a difference in clearance for nuclear weapons files. The US President can grant and revoke clearance for any document in government hands. That means that any file Trump took with him from the Oval Office is perfectly legal for him to bring.
Except for documents on nuclear weaponry. Those fall under a different, newer ruling, and those are the only documents he does not have full clearance over. As in, he can read them, sure, but he can't bring them along out of the White House. And that appears to be exactly what he did.
Now, did he go "meh, fuck it, I'm grabbing these anyway, what are they gonna do?"? In that case, he's a narcissistic idiot, and maliciously took files he knew he wasn't supposed to take, because he believes himself above the law.
Or, did he go "grab all the things!" and plain didn't know he wasn't supposed to take those documents? In that case, he's just a regular idiot. Ignorance of the law does not excuse breaking it, of course, but still. Not necessary malicious. Just par for the course, really.
Worst case scenario is the theory that the hotel in Mar-A-Lago is a popular visiting spot for Arabic oil sheiks. And that led some to reason that Trump took those files to sell to the highest bidder. If that is indeed the case, this just ramped up to treason of the highest order.
Personally, I think it's the second one. Or at least, that'll be the one the defense will put up. He might go around and shout "but I was president! I should have been able to do that!" to his adoring fans, who will no doubt lap things up, but that shit won't fly in court.