This is something I was thinking of (in the back of my mind) since 'across the sea'. I think the reason they can't kill each other is because they are already dead! This is also why i don't think anyone else can kill them either. As for Ben killing Jacob, I don't think it really happened (I think Jacob faked it; i mean why else would he let Ben do it so easily....and no i don't buy the free will theory).
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