Widmore wants Jacob and MIB dead

Maybe Widmore brings Desmond back to the island to release MIB from Jacob's cabin because he knows MIB will kill Jacob. Apparently, Widmore cannot kill Jacob directly or indirectly so he allows the only man on the island that has the will, desire and plan to do so. With Jacob out of the picture Widmore can focus on killing MIB so he can take control of the island since he knows about MIB's "Sonic fence" weakness.