Hi Lostpedia! Love the site. I'm sure I've visited it several times in the past as I've been hooked on Lost ever since season 2 (thank goodness my parents had Season 1 on DVD!). Great site. Great work here.
So, we've seen The Substitute. Here's some important points, some of which, after reading various reactions, I think may have been missed by many.
First, the Locke we know and love is dead, but he's not down and out. When MiB was chasing after the boy in the forest, then fell and started screaming "Don't tell me what I can't do!" This was very Locke, and very un-MiB. In fact, MiB was very smug when talking with Ben previously, discounting Locke as a man that was very confused. Well, unLocke had some very confused moments in this episode.... but returned to Sawyer confident and in charge again.
Next, who was the "Him" that MiB couldn't kill? Well, obviously it was against "the rules" to kill Jacob himself, but the boy's warning was for the future. The "Him" Locke was with was Sawyer. Case in point: When they reached the cliff, Locke could have easily pushed Sawyer off the cliff if he was allowed to kill him. He didn't do so. But when they began to climb down the cliff, MiB certainly didn't warn Sawyer about the ladder changes, insuring a safe trip down. MiB clearly wanted Sawyer to fall to his death here, but when Sawyer was hanging on within reach of MiB not saving him would have been tantamount to killing him, so he was obligated to help him out here. If this is the case, we can be sure he'll be trying to trick Sawyer and most probably all the candidates into dying one way or another soon.
I have to make some assumptions for the above reasoning; for example, it's within the realm of possibility that MiB has just been on and around the island (and going up and down those ladders) for so long that it didn't seem worth mentioning. Or he orchestrated the ladder incident so he could save him and be more trusted. However, these alternate ideas require too much of a stretch for me. For example, if in my house the entryway into a room was extremely low and required ducking, I'd still remember to warn a friend I was with to watch their head. And orchestrating the ladder incident only to save him would require him to know that Sawyer's hand wouldn't just slip and he'd fall to his doom. Jacob/MiB may have been in a time loop and have previous foreknowledge of certain events, but if so they're out of that realm now, as clearly seen by MiB being happy he finally killed Jacob and being surprised at the boy in the forest (this is the first time these events happened).
Whoa, sorry if I lost you in that last paragraph. I'll continue on with some other significant events.
MiB implied the cave was Jacob's when he pointed out the list. MiB's familiarity with the cave, and already seeing Jacob's residence, I think hints that it's actually a playground/residence of MiB. The list is the candidates the MiB learns about from Jacob's list. He writes them on the wall when he learns of them, then crosses them out when he orchestrates their death (or somehow they lose candidacy). This leads to...
MiB is conning Sawyer. In his previous chat with Ben he said how he felt about the Losties. "None of them realized how pathetic their previous life truly was", except for Locke of course. But now he tells Sawyer that their presence on the island, as orchestrated by Jacob, was "pointless" and "let's get off this island", the opposite of what he was previously saying to Ben.
But Sawyer's a great conman, and I wouldn't put it past the writers to have Sawyer involved in a long reverse con. Exhibit A: Sawyer's "Of Mice and Men" speech in the forest. Tipped off by Richard, maybe Sawyer even realized he wouldn't just be able to run from MiB and be safe. But let's pull a gun on him to illustrate distrust, then decide against it and continue on. The "Of Mice and Men" speech I'm sure was indicating that he thought MiB was taking him out into the forest to dispatch of him; personally, I feel that's a better interpretation that telling MiB that he was about to kill him. Come on! This is Sawyer! If he wanted to kill him, he would have just shot him. Instead, he wanted to make MiB think he was slowly gaining his trust.
A lot of my theories involve speculation, but really, they kind of have to at this point. We're getting answers, but we're still in the realm of "not enough information" to make an accurate conclusion. But hey, the ride is nice!