Mind you, my best ending allows for lots of unknown elements to take us there, lots of originality I'm not trying to restrict. But I think the best solution at this point is to embrace the "we are all good and bad theme" and this cannot be done with the defeat of the MiB. I like the idea, for example, that the MiB's name is also Jacob, that they are the dark and light side of one being (the island, duh). This would make the rules less arbitrary since it's a LOGICAL rule that you can't kill yourself and still live, but if you split the universe in two, so there are two copies of yourself... you can.
Thus the ending to this would not be the defeat of MiB, nor obviously of Jacob, but of the defeat of both of them. This creatures toys with us, MiB and Jacob are like the old men in Trading Places. The candidates need to defeat them both, and ideally they are the ones laid to rest, finally torturing no one's whole life to torment. Too bad Eve is female... that's the only glitch. Jacob is too cold to "win" as well. He's not "evil" but he is the sort of guy that'll distract you so your love gets killed. How victorious can defeating MiB be when that's what you do... you're not evil because you only do evil for good?
Ultimately... both these people should be defeated and the world is freed from these psychos.