In this place is death, "Christian" tells Locke that he must die after turning the wheel. When Locke tries to get up and asks him for help, MIB says "No, sorry, I can't". I think this is because MIB would be indirectly helping Locke to his death if he did this: by physically touching Locke, MIB would be helping him get to the wheel, leave the island, and ultimately die. Now, since Locke was a candidate at the time, MIB could not kill him because of the rules (just as he could not kill Sawyer because of the rules in The Substitute). I think MIB wanted to ensure that Locke died by making sure the rules could not in any way prevent John from dying if MIB touched him. Any thoughts?