Jacob v. Nemesis = Ben v. Widmore?

Ok, I'm sure someone has posted something along these lines before but there's a major similarity between the Jacob/Nemesis and Ben/Widmore relationships. Nemesis tells Jacob that he wants him dead but that's not possible for some reason so he ends up spending years figuring out the loophole to get it done. Compare that to when Ben visits Widmore at night and Widmore makes a remark on how he figured Ben was there to kill him and Ben responds "we both know that I can't do that". What is about these two pairs of rivals that they both can't kill eachother (at least by normal means)? Any ideas on a possible connection?

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