Sorry for the silly idea, but i think it's possible..

What if Des is kinda similar to biblical Christ, who knows or somehow predicts the future...
Well, what i want to say is that Christ was sacrifised in order to save whole humanity. He also new what will happen next...

It seems to me that Desmon knows the future too (the scene with Hurley improved it inevitably). What makes this fact possible is the scene when he woke up in the jungle. He was absolutely naked, he knew about the speech that will be told by Lock and finally at the end of the serie he looked like Biblical Jesus throwing the stones away wearing Hugo's T-shirt...