Loved the end but can anyone clear this up for me?

So Jacob told Widmore to bring Desmond to the island to unplug the light and destroy the island, hopefully bringing the MiB down with it incase he killed all the candidates. But if the island was destroyed then so would the light and thats the exact opposite of what Jacobs trying to do. Or was his plan all along to temporarily to unplug the light so MiB could be human and be killed? And did the MiB know that unplugging the light would make him mortal, cuz he seemed pretty shocked at bleeding. But obviously the light was the thing preventing him to leave the island so was he planning on becoming mortal and leaving? And why did MiB have to kill Jacob to leave, Jacob wasnt protecting the light he was sitting around making tapestry in the foot. MiB could have just walked right up to the light with Desmond, no one was protecting it?