Place the crab in the freezer for 2 hours or until it stops moving. Remove the legs and crack them with a rolling pin. Remove the top of the shell and clean out the insides with your fingers. Chop the crab and top shells in half.
Heat a medium saute pan over medium heat. Add the coriander seeds, cumin seeds and peppercorns and stir constantly until the seeds become toasted and fragrant, 3 to 4 minutes. Cool completely.
To make the paste, grind the coriander seeds, cumin seeds and peppercorns with a pinch of salt using a mortar and pestle or a spice grinder. Transfer the spice mixture to a food processor and add the vinegar, coconut, ginger, garlic, chile and onions and blend to form a paste.
Heat the oil in a heavy, large, wide pot over low heat. Add the paste and saute until fragrant and some of the juices evaporate, 5 to 10 minutes. Add the tomato, curry leaves, turmeric, chili powder and a pinch of salt and cook until the tomato pieces are tender and melting into the sauce, 5 minutes. Add the crab and 4 1/2 ounces water, increase the heat to medium and bring to a boil. Reduce the heat to low and simmer or until the crab pieces are cooked through, 10 minutes. Mix in the coconut milk and simmer for 2 to 3 minutes to allow the flavors to blend.
Divide the crab pieces and sauce among shallow wide soup plates and serve immediately.