Bear with me, I know this sounds ridiculous.
I think the enemy is
If Claire was not the one to raise Aaron, he was destined to be very dangerous. We know this from season 1, and he wasn't raised by Claire, this much we know. He wasn't raised by Kate either, ultimately it was weirdo Mrs. Littleton who raised him.
I believe that in Aaron's future he goes back to the island, and travels into to the past (maybe he becomes the leader of the 'Others' or something), and lives on the island with Jacob in ancient Egypt in some sort of metaphysical state, until they have some kind of despute and Jacob somehow becomes the grown-up physical form of Aaron (he kind of looks like him - blond, blue-eyed, a little bit retarted looking) - leaving 'Aaron' formless and bodyless. Thus he becomes ... the Black Smoke Monster. And is only able to inhabit dead bodies on the island. (Yemi, Alex, Christian, ... Locke.)
I think he can also inhabit these bodies OFF the island ... thus 'Christian' visiting Jack in LA or 'Charlie' visiting Hurley. Remember, Hurley says to Jack: "Charlie had a message for you too. He said you're not supposed to raise him. You think he means Aaron?"
I think the bearded, black-shirted dude from the finale of season 5 is just some random guy who was dead, and 'Aaron' used his body to speak with Jacob. He wants to kill Jacob because he wants his body back. And he wants sole supremacy of the island. Yes, I am still talking about sweet little 'turnip-head.'
But the loophole prevents this.
When the Black Smoke Monster becomes people, he does not have direct physical contact with them. (Correct me if I'm wrong). Unless he is in the form of smoke, when he usually beats them to death, or drags them across the ground. Remember when Christian Shepherd refuses to help Locke after he falls. Maybe that's a clue. Maybe he can't touch him.
But then I remembered that it was Christian (Claire's dad) who persuaded her to leave Miles and Sawyer and the gang and come creepily away with him, hanging out in the Cabin. And then I remembered ... this ...
And it got me thinking. HE (Christian, AKA Black Smoke Monster, AKA the guy who is in Locke's body AKA Jacob's Enemy) could touch Aaron. Because ... he was Aaron.
I need to rewatch season 5 and see if post-dead Locke physically touches anyone. That would be a huge clue.
But if this is right... this is awesome.
The last episode of season six could start with Aaron growing up with Mrs. Littleton and go on to tell his entire story, up to adulthood, Island-life, time-travel, Jacob hating, becoming the black smoke monster, and close the Loop of LOST !