I've been thinking more about the episode title "Reichenbach".
My current theory is that, if you see Dean as both Holmes and Moriarty, human!Dean (=Holmes) fought and beat demon!Dean (=Moriarty).
He subsequently faked human!Dean's death, for reasons as yet only known to himself. Maybe he has a cunning plan to take out Crowley eventually. Maybe he's tired of fighting everyone's (low) expectations of him. Maybe it's just easier to go along with it and pretend to be really evil.
Because, come on, how is it worse punishment to let Cole live than to slaughter him? Like, seriously.
Even though there is a very definitely more violent side to Dean, he has - as yet, and as far as we know - not mindlessly killed innocent civilians. (Well, unless you count the demon vessels, but then one needs to question Sam's integrity as a human being too, seeing how he cut up that girl in the first episode...And okay, if you count Lester, let me add that in a real life situation I would have severely frowned upon Dean killing him, but this is a TV show and sometimes I let myself cheer if douchebags get their just desserts.)
So this is my theory. Dean is faking having chosen the demon side. The real Dean is in hiding but still calling the shots from behind the scenes so to speak. I mean, just look at his sad eyes. Does that look like a cold-blooded demon to you?
But - this would actually be the first time that one of my theories have proven right, so I'm enjoying it until it gets disproved :D
And please don't ask me why Moriarty is still alive in this scenario. Does everything have to make sense all the time?
P.S. Just in case anyone feels like answering - please bear in mind that I'm a spoilerphobe and don't mention anything spoilery.