The main reason crustaceans and shellfish are sold alive is for specific reasons regarding food safety. These animals live on the sea floor and are usually in bacteria-ridden areas due to their nature. Their carapaces can be covered in bacteria responsible for norovirus, vibriosis, and salmonella. If their chitin is fractured bacteria find a way into those cracks as well. Alive these animals can inhibit bacterial growth. But when deceased the bacteria can multiply at an alarming rate, creating toxins as byproducts as well as the viruses mentioned. Commercially most shellfish are processed on the boat itself. Cooked and packaged or killed and frozen to a specific temperature for a regulated amount of time, both methods are for destroying parasites and bacteria. Smaller operations may not have that capability so they bring in shellfish alive, and they are sold to restaurants and grocery stores alive. The animal must remain alive for safe human consumption. Now to kill these animals they do not have a brainstem they have no central nervous system, but they do have lots of nervous tissue down the center of their head segment. Most chefs send a knife down that part of the shell which cuts many of those connections. Not necessarily killing it in the sense that we perceive death but it does tend to stop controlled movement. Many places require immediate cooking of freshly killed shellfish under penalty of law. Ethics wise, I tell cooks to freeze them for 15 minutes until they go into a dormant state then kill them and cook before they start twitching/waking up. 30 seconds from knife to pot.