Pretty obvious that Michael was sent by the Others to free their man, probably in exchange to let him see Walt or something like that. All these other theories are rather silly.
By the way, when you are going to make a statement like that, which in some cases (zombies, purgatory, clones) is true, be sure to not say something incorrect yourself.
If it is obvious that Michael was sent to free Henry, then why does Henry keep saying that the Others will kill him anyway since he failed? Also, why does Michael need to kill AL if he already has the gun and the combo, especially since we have seen him lock people in the armory already? Also, why did he bother staying around the hatch (we saw him in the previews) if he was just there to free Henry?
To me, it is pretty obvious that killing AL was one of the things he had to do since he said he was sorry and thus didn't want to do it. We have no idea at all what his intention are with Henry. He could have shot him or let him go.