Ok, first of all, this is not a "It's all in their head" theory, which is why I felt the need the add the "(Metaphorically)" in the title.

Some of you will remember that towards the end of 1x1, Walt drives the RV into a ditch, and then hears emergency service sirens. He first plans to fight the police, but realizes he could never do it. He then turns the gun on himself

http://i.imgur.com/5kDn3iQ.png

Walt doesn't just put the loaded gun to his head and give up. He pulls the trigger. Fortunately, the safety was on, but he didn't know this. For all he knew, when he pulled that trigger, he was going to die. He was ready to kill himself.

Walt then accidentally turns the gun safety off and fires a bullet into the ground. A frustrated Walt then gives up, and raises his hands for the police. But the sirens were not coming from a police car: they were from a firetruck, coming to put out the fire started by Amelio's cigarette.

Walt then realizes that he got away with it. He created a schedule I controlled substance and killed two people (or he thought he did, at least), and he got away with it.

After this, Walt knows he can get away with more than he thought he could. All his life he was living in fear of consequences, but something dark was always there in him, waiting to take over. After he "shoots himself", Walt as we know him is dead, and Heisenberg is awake.