A BROTHER?

Evil begets evil.

Without the time travel arc, where tortured soul Sayid kills a still innocent child in young Ben (who is taken to the temple and reborn as "a different" person, "one of the island") we could assume that Ben would have grown up as a bookish, meek man (like in the sideways teacher arc).

But instead, Ben turns into an evil, angry, vengeful tyrant.

But if the island "saved" Ben thorough the temple waters (and water was a means of summoning a smoke monster) one could resume that Ben was transformed into a smoke monster.

When Jacob killed his brother, the smoke monster was released from the light cave. We assume that MIB was the manifestation of Jacob's dead brother as a smoke monster. That is how MIB was created, from the waters of the island. Likewise, we can assume that since Ben was taken to the temple for a water ceremony (such as what happened with Sayid), then Ben would have been created into a smoke monster as well (and perhaps a better representation or brother to Jacob).

Ben was loyal to Jacob like a younger brother would have been under normal circumstances. But like a younger brother, Ben was upset that his younger sibling did not give him the attention, admiration and acknowledgment he thought he deserves. That was the focal point, the weakness, that MIB used to kill Jacob.

It makes some logical sense that only another smoke monster could kill another immortal entity on the island. But since the Crazy Mother's law that Jacob and his brother could not harm each other, that "loophole" was Ben.

MIB would have had to created a vast, complex and long term plan to even reach the loophole stage. He would have have spent centuries trying to find the right people to use, manipulate and sacrifice to get his "loophole." MIB knew about the island's special powers, and what it would take to create a smoke monster. So, by allowing humans to work on the island, try to tap the potential of the island energy, he got the humans to create an inexact time travel machine which the mixed up 815ers go back to encounter there nemesis, Ben, when he was an innocent boy. Already mixed up and confused, a time traveling Sayid was the perfect "assassin" to put MIB's final plan into action: shooting Ben gave the opening to create a smoke monster who would become Jacob's "assassin." It would have been the perfect check mate after a long game of cat and mouse between Jacob and MIB.