Saying they "have every right to" means nothing.
Ok, try "legally entitled". And it's not about making laws, I'm talking about international law here.

is it humane?
Not really.

Is it smart?
I can't imagine so, it'll cost him votes.

Do you want to be an isolationist ethnostate that has very limited trade partners?
I don't think this will happen. America controls the global economy because the dollar is the de facto petrocurrency and the USA will do whatever it takes to keep it that way, including enforcing regime change in disobedient states.

Have you considered that it's possibly not smart economically to treat people from neighbouring countries inhumanely
See above. There isn't a single nation in the world that will emerge the victor in an economic war with USA. You can hurt USA by refusing to trade with them, but they'll hurt you more.

and the migrants that do come from that country are a massive net positive for the economy and commit crime at a lower rate than natural born citizens?
This is debatable, and neither of us are well placed to judge the merits of this argument.

What is the point then of torturing and killing these people simply for crossing a border?
idk, I'm not the one making such decision.

It is not economical, in fact I'd be surprised if it isn't disastrous for the economy.
Well that depends. If it reduces the state's welfare costs while simultaneously freeing up jobs for American citizens, then it will be a boost for the economy. I don't know if that's what will happen, but you don't know it's not what will happen either.

It has nothing to do with crime.
That depends if you consider being an illegal immigrant as a "crime".

It's not because of religion, not that that wouldn't be a bullshit reason, but the vast majority of mexicans are christians.
Obviously religion ins't a motive when it comes to Mexicans, but you acknowledge it's a motive with regards to some immigrants in your next comment...

Insisting that Obama is a nigerian muslim and therefore an illegitimate president, signing a "muslim ban" for no other reason than to ban people based on faith... all the way to concentration camps for brown babies.
The fact they are "brown" is not the point. In the opinion of the state, they are not American, that is the point.

You can either jump through hoops to say correlation is not causation for every single one of those, or you can accept that he's a fat racist retard who's so incredibly dumb that he can barely speak his own language.
I mean, this is just way too simplistic. This is politics, whatever the motivation it will be viewed as politically advantageous by the powers that be. I'm not pretending to understand the motives, but I'm not going to default to "racist fat twat". Perhaps I'm wrong.

This particular girl never was an illegal alien. You can read the story. I wouldn't care if she was, but in this case I think you'd have a pretty tough time calling this anything but ethnic cleansing.
That's extremely powerful language. That's worse than simple dumbass racism. I don't believe for a minute that's what's happening. What enthnicity is being cleansed? Brown people in general? USA is home to many of the most successful black people in the world. Do you think that is going to change if Trump remains in charge?