I like this theory.. Ben betrays the betrayer.. Remember what Ben said to Locke in The Man from Tallahasee..

The sub keeps the ILLUSION of his people being able to leave..

Also that Locke did him a big favor by blowing up the sub since he didn't want to let Jack and Juliette leave... He could not go back on his word, and letting them leave showed weakness.. I think he said..

So I doubt Ben just let Walt and Michael Leave.. Good Point

__________________
Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.