I'm fairly certain that this has been discussed before, but I couldn't find it. I'm not talking about the vague "end of the world" circumstances that Widmore claimed, but what it would mean for mankind.
Would we go through some kind of tribulation period? Perhaps one where the MiB would be able to fulfill his desires and allow his followers to do so as well. Maybe it would just be a gradual change into hell.
So some thoughts/speculation on how the MiB could bring about the end of the world. Perhaps he could take on Hitler's form and terrorize the world. Regardless, we're all screwed if that were to happen. Thank you Jack and Desmond!