Obviously the Man in Black knows who all the candidates are. So I propose the idea that we have seen him try to kill some of them indirectly before season 6. Let me give 4 examples:
Jack: In 1x05 Jack's dad appears to him. This is revealed to be the Man in Black (and confirmed by Darlton). Jack follows "Christian" to the point where he falls off a cliff only to be saved by Locke. Perhaps the Man in Black was trying to lure Jack off the cliff thus indirectly killing him.
Hurley: In 2x18 Hurley starts seeing his imaginary friend Dave again. "Dave" tried to get Hurley to jump off a cliff claiming that committing suicide in this "fake world in his imagination" would bring him back to the real world. Well I propose that Hurley wasn't seeing his imaginary friend he was seeing the Man in Black in the form of Dave and he was trying to get Hurley to kill himself cos he knew he couldn't kill Hurley because Hurley was a candidate.
Nikki: Thanks to user SuperJar for this one. In 3x14 we see the deaths of Nikki and Paolo. Nikki kills one of the spiders which attracts more of them. Nikki dies because she is bitten and paralyzed by the swarm of medusa spiders and then buried alive. It was confirmed by Darlton that these medusa spiders were a manifestation of the Man in Black. So he could bite her knowing it wouldn't actually kill her. What would kill her would be the other castaways burying her alive. (We know she was a candidate as 'Fernandez' is one of the names crossed off in Jacob's cave).
Sawyer: This one is a little tenuous in that it's hard to explain how the Man in Black is trying to indirectly kill Sawyer here. But the boar in 1x16 that annoys Sawyer could well be the Man in Black trying to lure Sawyer somewhere like what he does with Jack in 1x05 which fails because of Kate's input. Just an idea.
We also see a horse appear to Kate in 2x09 and I have often wondered if that was the Man in Black. If so, why? Any ideas on that one?
Give me some thoughts on this theory though, it would be much appreciated =)