I know this thread is a little old, but I am going along with it anyway.

The possibility that Desmond is lying is very, very high. Desmond told Charlie that he saw Claire and Aaron getting onto a rescue helicopter; since Charlie's death that hasn't happened and Aaron has already been rescued. So if that vision alone was a lie, did Charlie have to die to save Claire?
Honestly, I believe that Desmond was tired of protecting Charlie and simply let him die.