I think they're hurting on the inside.
My theory? The Key turning was a save point. Eventually Desmond will be faced with losing Penny forever to save everyone else. Jack will off himself, Kate will be killed, and everyone on the island will be threatened. That creepy chick will come back and offer Desmond that he can return to that point, but the Island will be removed from the Earth forever.
But mainly Charlie died because they weren't using him as a main character anymore and it pissed the actor off. What's terrible is, in the season 3 DVD's bonus thingy, there's an awesome scene of Charlie chopping wood after Desmond screams "YOU'RE GONNA DIE, BROTHA!" and he's all emo and shit. Kate comes out of the woods, and offers him to go with her, Sayid and Locke to the flame (why he didn't go is beyond me), he refuses and she kisses him on the cheek. So sweet.