I was always told that the reason lobster is usually sold live is because bacteria in the lobster starts to multiply quickly after they die and the meat will become unsafe.

Cooks have told me to either run a knife through their brain or throw them in a freezer for half an hour to shut them down.

Incidentally, I used to do the latter as a kid when catching bugs outside to feed a green anole I caught and was taking care of for a Boy Scout merit badge...