i think desmond knew all about the code having to be the beach boys song and that mikhail would blow the window and it would flood the looking glass comms room. he knew that only charlie could do it and only way he could get charlie to do it is if he lied to him saying that if he did it, it would save claire and aaron.

but i rekon that since every other attempt to kill charlie has failed, he thought that he could save charlie again. desmond seemed to know straight away when he saw mikhails body was missing exaclty where he was, and what he was going to do (desmond didnt even look around to see if he was hiding - just ran straight for charlie. So really charlie didnt have to die (if he is dead) and desmonds plan backfired.

And does this mean that mikhail is dead (again)?