Flight crews cannot declare a passenger dead. Therefore it is a medical emergency until a qualified medical professional does so. A physician on board might do so, but that gets muddy real quick legally.
Also, the diversion for a likely dead person isn’t for the dead person, it’s for the family that would sue the airline for carrying on for however long to the destination. They’d argue whether or not the person might have had a chance had they diverted. So legally and financially an airline will try to get seriously ill or potentially dead people off the plane as soon as practical.