The mob didn't know at first what Roberto Bernal had done, but he was running and that was enough. Dozens of men loitering on the sidewalk next to a supermarket kicked and punched the 42-year-old until he was bloodied and semi-conscious. After all, they had been robbed of cell phones, wallets and motorcycles over the years, and thought Bernal had a criminal's face. Then a stooped, white-haired man trailing behind told them he'd been mugged. The mob went through Bernal's pockets and handed a wad of bills to the old man: The equivalent of $5. They doused Bernal's head and...