You know, even Hitler did not publicly say "I'm ordering all Jews killed." (True, he did in his notorious January 30, 1939 Reichstag speech say that if the international Jewish financiers in and outside of Europe plunged the world into another war, the result would be the "annihilation of the Jewish race in Europe." But even here he does not explicitly say how this result will come about or "and I will be the one to do it.") In the US an order for mass annihilation cannot be kept secret. And this is a nation where even the relocation and internment of the Japanese-Americans came before the Supreme Court--true, the Court (unfortunately) sustained it but at least it considered it, heard arguments from lawyers by both sides, etc. The basic problem with the OP is not "FDR wasn't that evil" (though indeed he wasn't) or even "FDR wasn't that crazy" (though again he certainly wasn't). The real problem is that even if (due, say, to some bizarre stroke) FDR does want to do this, he can't get away with it.
BTW, remember that even of the nationalities deported by Stalin (in an infinitely more brutal process than that of the US with Japanese Americans) the majority of the deportees survived. (E.g., "Estimates of the number of deported [Crimean] Tatars range from 180,000 to 250,000 and it is estimated that between 20 percent and 40 percent of the deportees died en route or within the first year after resettlement." https://www.du.edu/korbel/hrhw/researchdigest/russia/repatriation.pdf)
Excacty, FDR ordering the mass execution of Japanese nationals and Japanese-American US citizens is beyond ASB.
The mass internment of several hundred thousand people based on ethnicity is about bad as it gets in the 20th century US, but does not even approach the level of violence seen elsewhere.