Louis Farrakhan did not actually endorse Trump. Actually, he wrote on his Facebook wall recently that a Trump presidency would make the country to go hell or some such nonsense.
Trump’s seeming encouragement of violence among his supporters is the main thing that troubles me (since there are certainly parallels with Mussolini’s blackshirts and Putin’s thugs). Building a wall with Mexico is actually a pretty reasonable security measure and administrative policy (Hillary was even for it at some point).