In terms of the TPP, I’m pretty sure the main guiding principle was that he said he’d back out of it since it played well with the protectionist base and that he’s obsessed with raging on multilateral “deals,” especially any made by Obama, who he’s childishly demonized for the last eight years. The biggest losers in the TPP were Chinese industry, the rights of individual Americans, and the little people screwed by “trade” agreements that enabled more capital flight - none of those were noted by Trump. He primarily claimed killing it would help American jobs, where its biggest problems were largely orthogonal to jobs, in the short term not passing TPP will kill maybe a thousand or so US jobs (largely built on the expectation of its passage and opening Japanese and other markets), and in the conservative long term estimates noT passing probably cost the US 10-20k (crappy) jobs.
So far my working theory has been ‘simple-minded.’ His stream of fumbles, political failures, and incoherent rambling seem to back that up. Smart presidents don’t rant at ego-boosting rallies about a fake disaster in Sweden they heard about on Fox News, then double down when the majority of people are well aware they’re BSing. He’s mostly surrounded by B-listers. Bannon is evil but has a middling intellect. Conway, Miller, Kushner, Carson, and many others around him are embarrassingly stupid. Even Pence is no rocket scientist, even if he’s more cunning and talented at effecting his evil plans. Beyond that, he’s set hist staff against each other in competition. The one saving grace of this horror show so far has been that they’ve been really ineffective at using the many powerful tools at their disposal.
Maybe I’m wrong that he’s a dolt, but he and the people around him are dangerous regardless of whether they’re bright or not, and really far more dangerous if they’re as ignorant/incompetent as they seem.