During the finale of Lost, The Man in Black travels with Jack to the Source of the Island's powers in order, in his objective, to destroy the Island and finally get away from it.
However, Jack's plan was to use this opportunity to finally defeat the now-Mortal Man in Black and prevent his departure from the Island.
Did the Man in Black know he would become mortal when the Island's power was removed, and was willing to risk this vulnerability in order to escape, or did he simply have a fatal oversight on his plan?