Just had a thought. When Ben visited Widmore to say he was going to kill Penny, Widmore asks "are you here to kill me Benjamin?". Ben replies with "we both know I can't do that Charles.". Now most people think that this is because of "the rules" as if they were rules in a game, but what if it was a whatever happened happened thing. Maybe they both know when and how Widmore dies, and it isn't Ben that kills him, and no matter what he does Ben will never be able to kill him. Much in the same way that no matter what Michael did he couldn't kill himself because he was meant to die on the freighter. Remember Ben didn't say he couldn't kill Widmore because of the rules, he only said he changed the rules by killing Alex.