In a bowl mix the ketchup, chili paste, oyster sauce, soy sauce, and tamarind paste; thin it with 1/4 cup water and set aside. Pour the oil into a wok or large skillet over high heat. Add the scallion, ginger, garlic, 1/2 the chile, and cilantro and cook for 1 minute. Add the crab and stir-fry for another minute. Pour in the sauce and continue cooking, stirring often, until the crab has absorbed the sauce and the sauce has thickened, about 5 minutes.
Put onto a platter and garnish with the rest of the chile and cilantro. Serve with plenty of napkins!