Here's the most generous take I can come up with:
One of the myths in Republicanism is that Obama ran off to Chicago with millions upon millions of documents. And it's true, he did! He absolutely did. That probably made Trump think he could and should do the same, and that the National Archives and DOJ were just being partisan when they told him to give the documents back.
What Trump is missing, because the Republican myth ignores this, is that all of those Obama documents were cleared by the National Archive, to go to the Obama Presidential library, where they are still maintained by the National Archive. And that's standard procedure. Trump didn't get the documents cleared, and he has no Presidential library. He just grabbed a bunch of documents and took off. And he thinks that fine because he doesn't live in reality; he lives in the Republican mythology, where the tales say Democrats get away with everything, so he should feel free to be just as lawless.
A President can't actually unilaterally declassify nuclear secrets anyway. They always have to go through DoE, first.