MIB sank the island by supernerd

First off, let's look back at the episode, LA X.

In the conversation between MIB and Jacob's "body guards" MIB says that since Jacob's dead, they don't have anything to protect, therefor, they can leave, they're free. He later says that he wants to go home.

Now, in earlier seasons at many times, multiple people refer to the smoke monster as the island's guardian, it is protecting the island.

Now all we have to do is put two and two together.

MIB WANTS the island destroyed so he can be free. He is a slave, being forced to guard the island. He wants, or needs, someone to sink the island so he can be free.