I'm sure people have already thought of this so call me behind the times if you'd like but I just realize this upon watching Happily Ever After again.
Widmore told Desmond he is going to have to make a sacrifice. If he doesn't, everything that they have been through would be for nothing. He hopes for everyones sake, Desmond agrees to this sacrifice. He tests Desmond to see if he can withstand and survive a large amount of electromagnetism. Widmore's goal is to kill the Monster. Therefore, the way to kill it must somehow be related to electromagnetism. Think about, the pylons keep the Monster at bay because the use some form of electromagnetism. Desmond is going to have to somehow do something with the Monster that involves electromagntism and since Desmond is the only one who can withstand it, it has to be him. (Did anyone see Heroes season 1 when Claire had to be the only one to shoot Peter because she was the only one who could get close enough to him.) So basically Desmond has to kill the Monster and that is his sacrifice and if he doesn't do that, MIB will live. That is why MIB tried to kill Desmond and is why he asked Sayid to kill him so that Widmore couldn't get him back so that they could kill the Monster.
This is my theory but I'm sure everyone else already thought this so I guess I'm alittle late in the game.