I think Mr. Ecko has a vision in Johns sweat lodge and that's what we see catching on fire in the promos. Charlie/Sayid and hurley pull him out. But, when he get sout he is not the same person. He's had a vision of Emmie who tells him it's time to die. They show in the promo sneak peek that he disappears. So they go lookin for him and when they find him he is different. That's when he attacks Locke

If he's the one hwo dies then that's how he dies. Someone has to kill him to stop him from Killing Locke. The cost of Living.