[MegaPrime33]

MIB can't kill any one who is a candidate. Apparently that is one of the rules. But, if they all die on their own, then he can leave the island. He set the bomb so that it would only go off if it was tampered with. If Sawyer trusted Jack, nothing would have happened.

I'm not sure that was MIB posing as Walt when he appeared in front of Shannon, and then later Sayid as well. And I dont believe MIB needs the person to be dead in order to take their form, however the only example I have of this is Walt showing up in front of Locke, after Ben shot him. When he appeared before Shannon, he was soaking wet. We learned that water now plays some role with the MIB, after Sawyer told Jack to push MIB in the water. I'm curious if that affects him in some way.