I don't know if someone else has posted this already but here goes - I saw in Ab Aeterno that MiB told Jacob that he would kill him and escape from the island. And soon after Jacob leaves, he breaks the wine bottle and and all the wine spills out. I think it was pretty clear that Jacob represented the bottle and hence all the candidates too would be like layers of coating over the bottle. I believe that MiB's mission is to kill all the candidates so he can leave the island for good.

This is similar to what happened with Dogen. When Sayid kills Dogen, Lennon tells him that"He was the only thing keeping it out." After Dogen's death, MiB was able to enter the temple and kill everyone.