Please forgive me if this is not posted correctly, or if it's been discussed....I just joined this board, so may make mistakes at first.
In Season 3, "The Brig", Sawyer pins Locke in the jungle and asks him why he doesn't kill Ben himself. Locke screams in frustration that he CAN'T kill him. Could it be that Locke was "claimed" by the MIB/Nemisis/Smokey when he "looked into the eye of the island....and it was beautiful" in Season 1? He could not be fully possessed until the copreal body died....but we know he was taken over then. Could it be that, since the rules state "you can't kill him" (them? the candidates? the claimed?), that the early Locke had to follow the rules, even though he was still, technically, Locke?
This still works if Jacob is the one who brought Anthony Cooper (the man Sawyer ends up killing in revenge) to the island and claimed him....then, Locke could not kill him either...but Sawyer could. Is this why the current "Locke" chose Sawyer? Because he has killed for him already?