From "The Incident":
MAN IN BLACK: Do you have any idea how badly I wanna kill you?
JACOB: Yes.
MAN IN BLACK: One of these days, sooner or later... I'm going to find a loophole, my friend.
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From "Ab Aeterno":
JACOB: So you tried to kill me?
MAN IN BLACK: You expect an apology?
JACOB: No. I guess I'm just wondering why you did it.
MAN IN BLACK: Because I want to leave. Just let me leave, Jacob.
JACOB: As long as I'm alive, you're not going anywhere.
MAN IN BLACK: Well then, now you know why I want to kill you. And I will kill you, Jacob.
JACOB: Even if you do so, somebody else will take my place.
MAN IN BLACK: Then I'll kill them too.
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I'd say MiB doesn't need the candidates to leave with him. He would be fine just killing them all. However, it took him a helluva long time to find a loophole that allowed him to kill Jacob. I doubt MiB wants to go through all that trouble again to kill the Island's new protector, especially since he doesn't know exactly which of the candidates will BE the next leader. He'd have to kill them all, just to be safe. And ESPECIALLY since it appears MiB can't kill the new Island "protector" and the candidates himself.
So having all the candidates leave the island with him would seem to be a smarter solution. Because that leaves the Island leader-less and without a protector, so it's without anyone to keep MiB from leaving himself. And also, another thought: it might be that, once he leaves the island, MiB will have the power/authority/whatever to kill the candidates, but while they're on the island he may not be able to.
So why does he need Ben? Here's a clue, maybe, possibly lol:
From "Dr. Linus":
LOCKE: I'm gathering a group to leave this place for good. But once we're gone, someone's going to need to be in charge of the island.
BEN: Me?
LOCKE: I can't think of a better man for the job.
BEN: Well it doesn't seem very likely does it, under the circumstances. How am I going to get away from...
[Locke interrupts him by apparently releasing Ben's leg restraint telekinetically]
LOCKE: Come to the other island, to the Hydra station. That's where we'll be.
BEN: She'll come after me.
LOCKE: 200 yards inland there's a tree in a clearing with a rifle leaning up against it. If you go now you'll get there first, and you'll have the drop on her. But don't hesitate - she won't. See you soon, Ben.
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It could be that MiB wanted Ilana dead, but can not kill her himself for some reason. So he wanted Ben to kill her for him. However, he couldn't just come out and ask Ben to do it, so he gives Ben just the right motivation to do so himself: removes his leg shackle, promised Ben he will be made leader of the island (even though MiB has no real plans to have a leader on the island), and leaves a rifle for Ben to use on Ilana. MiB put all the chess pieces in place, secure in the knowledge that his ultimate goal--the killing of Ilana--would be the end result. But Ben didn't follow through.
Now, of course that leaves the fact that Ilana actually DID end up dying lol...just not as MiB planned it.