Hmm , this is probably already a highly debatable and topical subject.
But , Locke is obviously not dead , in the Paulo/Nikki centric episode , we were purposely introduced to the Medusa Spider and the paralytic like venom it excretes from a bite which lasts for at least 8 hours. We do not however follow Locke's body from it being deposited at the Butchers. Its entirely possible that Locke's body is somewhat in a hibernation state , preparing for his re-awakening back on the island.
So its entirely possible , Locke does not have to die , but instead perceiving the image of himself being dead , means they have to back to the island , while he is at all possible , asleep in the coffin with a heart rate lower then a doctor could hear ( something Arzt mentioned to Nikki ).
This is an idea , which im sure has been thought about by a majority , but wondering what you think and whether this is a viable , theory/option.