Okay, I have to write this down before I forget.
Okay so Charles Widmore was on the island 50 years ago. I guessing that he was the leader that Ben mentioned when questioned about killing the Dharma Initiative and there was some kind of struggle for leadership between the two of them and Widmore lost so he had to leave the island.
So here's my math, work with me here, everything is approximated.
Okay assuming Widmore is around 18 in 1954 and Ben Linus was born around 1965. So in 1970 the Dharma Initiative was founded. Widmore would have been around 34 and was the leader of the Others. Ben arrives on the island around 1975 around the age of 10. So in 1992, the Purge happens and Ben is 27 and Widmore is 56. Widmore is the ruthless person Ben talked about as the leader who ordered the Purge. At some point soon after, Ben and Widmore get into a power struggle and Jacob, I'm assuming, picks Ben as leader or whatever and Widmore has to go. So he leaves and becomes rich but is still mad about having to leave the island and that Ben took his place so he is trying to get back on the island for whatever reason. So now he is 73 and Ben is 46. How Widmore got to the island I don't know but I think that Ellie, the girl with the gun, is Daniel's mom. And that Widmore and she had some sort of falling out too (maybe about the purge) so that is why she is working with Ben.
That is my theory anyway.
I still don't know what's up with Richard Alpert though.