We know that MiB's loophole, seemingly, was getting Ben to do his dirty work, but couldn't he have been using "loopholes" all along, by trying to kill the candidates? A loophole is a way around a rule. If one rule is that he can't directly kill candidates, isn't trying to get the candidates to kill themselves, or eachother, a loophole? This isn't so much of a theory as it is speculation.
-When Jack is chasing Christian in the jungle, and nearly falls off of a cliff, we assumed that it was MiB in Christian’s body, however, it was most likely just an apparition of MiB’s (Like Isabella to Ricardus). He was trying to get Jack to accidentally fall off the cliff, but he can’t directly cause the death of a candidate.
-In this same episode, Locke gives Boone a hallucinogenic via open wound, and Boone explores the inner depths of his mind, which enable him to come to the realization that Shannon was hindering his progression as a person, and holding him back. How else could Locke have known that this precise method of psychotherapy would, at that moment, help Boone, if he hadn't scanned him for information already, and his previous memories? An even better question: How, if Boone has never seen the monster, was he able to hallucinate and imagine it, without ever actually witnessing it? Well, we know that the monster, with Richard, was able to make Richard believe that his wife (appearing to him as a hallucination) was being killed by the monster, so I believe this is the same exact case. In order for Boone to fully trust Locke, Locke had to prove to him that he didn't need Shannon anymore. Remember how Boone was the "sacrifice the Island demanded?" Perhaps it was the sacrifice that MiB/Loc! ke demanded, because, since Boone Carlyle (Candidate 226) is a Candidate, he can't kill himself, and had to willingly choose to go into that plane so that he could crash and die.
-Dave appears to Hurley and tries very hard to convince him that he is insane, and should kill himself so that this "reality" would end. Again, Hurley wouldn't be able to kill himself even if he tried, but regardless: Dave was MiB, as an apparition, trying to get Hurley to willingly jump.
-Walt appears to Shannon in a tent to get Shannon to follow her (out of curiosity/shock that Walt is "alive"). He subsequently leads her to her death, at the hands of Anna Lucia. This enables Sayid to channel some anger toward Anna Lucia, and perhaps some anger that will stay with him for when Locke needs to claim him. Needless to say, Walt appeared to trick Shannon into walking into her death.
-And finally, the most interesting, but mainly overlooked example: Sayid was given LSD on a sugar cube by the torturer in the Dharma Initiative "Oldham," and tripped hard. What happened when he came to? He said he knew what he had to do now. Perhaps MiB had influence over him, and considering the fact that Ben is a candidate, MiB subconsciously told Sayid to murder him as a boy. Another possibility is that MiB knows that in order for Jacob to be killed by Ben as an adult, he MUST be healed by the pool in the temple, perhaps to be claimed, or for other reasons, and the only way for that to happen is to subconsciously tell Sayid to kill him.