I don't think Trump even wants necessarily to pull out of everything international. He wants to pull out of everything Obama did.
The Paris climate accord was one of Obama's best accomplishments (even if it was too little, too late). So Trump withdraws from it.
The Iran nuclear deal was one of Obama's best accomplishments. So Trump withdraws from it, and then tries to claim that the deal he's made with NK is good, even though it's probably going to be something along the same lines as the Iran deal, only weaker.
The ACA was Obama's signature domestic policy accomplishment (never mind that it was a complete turd compared to what every developed country has). So Trump starts attacking it as he can through executive orders, and Congressional Republicans use the fact that Roberts upheld the mandate under the taxation clause rather than the commerce clause to repeal the penalty... which now has states suing over the constitutionality of the mandate now that there's no penalty (so it can no longer be a tax) and trying to get the whole thing invalidated with the support of the US Department of Justice.
Trump has two guiding "philosophies": be a racist, sexist ass, and undo everything Obama did.
And he basically thinks he's a king, which is why he hates leaders like Merkel, Macron, and Trudeau, because they actually respect democracy and the notion that the voters could turf them at the next election. Trump just wants to be absolute-monarch-for-life and thinks anyone who isn't is weak. It's why he likes Putin, Kim, and Xi so much--they're strong because they've found ways to be in power perpetually.
EDIT: For another example, look at the TPP. Obama was for it. Trump railed against it on the campaign trail (which may well have swung enough votes in the rust belt toward him). When Trump gets into office he withdraws from the TPP. But he was never really actually opposed to it, because a) now that NAFTA is being renegotiated some of the most atrocious parts of TPP are getting in there--but that's a Trump "accomplishment" and b) now that the other parties to the TPP have reworked it to account for the US not being in the deal Trump suddenly wants back in since it's no longer Obama's TPP.