On any given day, Elon Musk might be the hero or the villain. Call it the duality of Musk—part of the reason he confounds both supporters and critics alike with achievements that amaze and comments that appall. That duality has been on display again as he attempts—in his own way—to solve some of the hardest problems around: enabling people who are paralyzed and curbing illegal immigration. First, the hero moment more than a week ago: Musk’s brain-computer company Neuralink gave a generation of disabled people new hope that they might one day walk, gain sight or live a more able...