This is where I go outside my normal feelings on matters. Normally characters reactions and actions, as well as the question "How would I write the story?" or "How would the writers complete this as a good piece of literature?", are good indications to base theories on (or rather, to discount theories). Here I'll ditch my normal common sense.
For example, when Locke said to Richard "It's good to see you out of those chains, Richard" there were theories around saying the chains were symbolic and he meant because he's free of servitude to Jacob, but I find those theories a lot of hogwash, no offense.
Here's why: He said virtually the same thing to "Jacob's bodyguards" just before he became the smoke monster and killed them. Except that time he said (from memory) "You're free. Jacob's dead. You don't have to protect him anymore." Nothing about chains. So if that's all he meant when he was talking to Richard he wouldn't suddenly switch to metaphor and make a remark about chains. Also, Richard's response: "[eyes go wide] It's you." He recognized Flocke based on that comment about seeing Richard out of those chains. So Richard knows someone who previously saw him in chains. (And I bet 10 to one we'll get to see it in a flashback, but at the very least we'll learn the history.)
So, now I'm going to break from my normal common sense on such matters and come up with some more theories.
Theory 1: The Others can't kill candidates, but the candidates can kill each other (or themselves?). This comes down to the topic of Free Will vs. Destiny. You prove nothing in that argument if you just do out and wipe everyone out, or the Free Will side killing some before they can fulfill their Destiny. I'm not sure if outsiders (like Dharma Initiative) count as Others that cannot kill candidates.
Supporting Evidence: From Day One the Others weren't outright killers. Friendly said "The only reason you're living on [the island] is because we let you live on it." Yeah, right. A nice bluff because he couldn't kill them. Mysterious Boy reminding Flocke that he can't kill Sawyer.
Counter Evidence: Ethan killed Charlie (though Jack saved him). Mikhail killed Charlie. Dharma guys killed Sayid. Widmore guys killed Claire. Boone died. Shannon died. Ben killed Locke.
Counter-Counter Evidence: Charlie didn't actually end up dead when Ethan attacked, but this could also be explained by Charlie not being a candidate or Ethan also being a candidate. But if Charlie is not a candidate then Mikhail doesn't have to worry about killing him. Sayid and Claire were both killed, but this kind of supports the theory, because they came back, maybe because they were killed by non-candidates. Boone died through his own actions. Shannon died by Ana Lucia, so either Shannon wasn't a candidate or Ana Lucia could be one. Ben could kill Locke because Ben was a candidate at one point, too. (All the leaders of the Others, I think, have been candidates.)
Okay, that's a pretty crappy theory. Here's another one.
Theory 2: The Island is still a character. I think a lot of us have lost sight of the fact that the island was designed to be a character since Season 1. We kept hearing, too, "the island" wants this or "the island" won't let you do that. what if the island is still another entity? Perhaps the island is what enforces or governs "the rules" that Flocke must adhere to.
Supporting Evidence: After Jacob died, Flocke (and Sawyer) saw the Mysterious Boy. Some think he's another manifestation of Jacob's; Jacob is still able to talk to Hurley, after all. I say he's a manifestation of the island's, and maybe other visions have been also.
Counter Evidence: This could possibly be explained away eventually as all actions of either Jacob's or Smokey/Flocke.
Counter-Counter Evidence: Nuh uh!
Theory 3: This season is going to be awesome! PROVEN