Slaves (from a different country) are in the best interest of the people of that country. Thus acting in the best interests of your people can in fact be ethically wrong.

Are they? They're useful to individual slaveowners with enough capital they can have the slaves work. But what about for the working class? People in the U.S. today remark that immigrants willing to work for lower wages than locals drives down wages. A population forced to work at zero wage (or rather, at the cost to the slaveowner of keeping them alive and captive) would surely do the same.

It doesn't seem clear to me at all that, if I were the spirit of democracy/the hypothetical purely benevolent (to specifically the people I consider of my country/nation/tribe, to the possible detriment of all others) emperor of wherever, that on balance introducing slavery and slaves into my country would be a net boon on average.

Even ignoring the material aspects, it seems like americans had to contort their worldviews quite a bit to justify slavery to themselves, given their otherwise stated moral beliefs (that were useful to maintaining society). Something would have to give a pretty decisive advantage to justify that required cognitive dissonance, and I don't think 'enriching some already-rich landowners a bit more' is useful enough.

I mean, look at america. Are the descendants of the ethnic groups in the us at the time of slavery on average better off for slavery having existed? Now?

Plus it's wrong to inflict such horror on your fellow man etc etc, and there's a pretty huge difference between not letting people in and abducting them into your country and whipping them.