The enzymes in a lobster's (or crab's) digestive tract are quite potent and can quickly start decomposing the flesh once the creature dies. Unlike the digestive system of fishes, mammals. or birds, that of lobsters (or crabs) is difficult to remove before they are cooked. Another reason for a cook to insist on a living lobster is that live crustaceans are far more likely to harbor pathogenic microorganisms (such as hepatitis-carrying viruses) than are live fish. And when they do, the microbe count in the lobster will also likely be much higher. Those disease-causing agents, given a head start, will multiply at a much faster pace in a crustacean. Your best defense, therefore, is absolute freshness.
Even if you could safely cook and eat a lobster that has been dead for 24 hours, there would seldom be a way to ascertain beyond a shadow of a doubt that the uncooked lobster displayed in your local fish store had died a recent death. You can, on the other hand, determine whether the lobster is dead: Pick it up by its back. Unless the tail curls under the body, the lobster is dead of close to it.
** Asian Recipes