I understand that it is not safe to cook crawfish that have died. So if we must kill them right away how can we safely take them home to prepare for a meal?
By rule, it is illegal to transport live crayfish away from the body of water where they were captured. This is to prevent spreading of invasive crayfish to new waters and to prevent crayfish diseases and parasites from being spread to waters that may not contain the disease or parasite.
Crayfish do breakdown rapidly once they die. What I've seen crayfish anglers do is to hold their crayfish until they are ready to depart. At that point, they kill the crayfish and break-off and place the tails (the portion you eat) in a bag with ice. They can then be transported to their final destination for consumption.