What if Castiel went to rescue Sam from the cage, fought Lucifer and LOST and Lucifer used Castiel’s meatsuit to get out of the cage. So the Cas that we have had since is actually Lucifer.
This would explain Sam’s soul being left behind in the cage - Lucifer wanted to mess with things up top and not have real!Sam getting in the way.
It has been pointed out in show that it was useful for Cas’s plans for Sam to be Soulless. Sam working with Dean is twice as likely to screw up some bad-guy’s (Castiel in this case) plot, and Soulless!Sam kept his distance from Dean, so yes it would have been done on purpose.
Also Lucifer is a stronger angel than Cas and Cas shouldn’t really have been strong enough to get Sam out on his own (since it took a group of angels to get Dean out of hell) - but if Lucifer had the door opened for him, he might have been.
That would mean:
Lucifer was the one working with Crowley.
It was Lucifer who wanted the power from Purgatory (to fix humanity once and for all), not Cas / Crowley.
Lucifer could mess with resouled Sam because he (Lucifer) wasn’t in the cage, he was on the same plane as Sam - so Hellucifer never existed.
Breaking Sam’s wall was pretty unforgivable on Castiel’s part - and not really in character - if it were Lucifer doing it that would make sense.
Godstiel got to kill lots and lots of people - and Lucifer really wanted to be doing that.
All the stuff about Cas taking on the hallucinations? Well Lucifer is the prince of lies after all (Lucifer approx= Satan = Prince of Lies) so he could have been playing the long con all this time, with Crowley helping him out. Maybe Crowley knew all along it is Lucifer and not Cas and that is why he keeps helping him.
Lucifer is the sort to whisper in Dean’s ear about how everything he does is right (in fact the Mark of Cain storyline shows him doing exactly that to Cain) - that is what Cas has been doing. Cas's point of view about Dean's part in the possession affair, particularly during the scene where Sam is being tortured by Crowley is puzzling because Cas was always pretty judgemental even of Dean. But his saying that Dean wasn't doing anything wrong here definitely added a level to the problems the brothers had afterwards.
Lucifer would also whisper in Sam’s ear about how not-worthy (2nd biggest screw-up and all) Sam is.
His way of getting around both Sam and Dean then would reflect what is most likely to succeed with both of them, and doing it in the guise of Cas would be really smart.
And now Dean has the Mark of Cain (from …. wait for it … Lucifer) and we have come full circle. Cain suddenly appears in the picture just when Lucifer is ready to cause more trouble? And the plots of the past few seasons might actually hang together a bit.
Also Metatron seems to have the hots for Castiel, but wouldn't it be more likely that he would want to be the sidekick of a hotshot arcangel than a lowly ‘brother’ angel.
On the plus side for other characters that would mean Castiel is in the cage presumably protecting Adam from Michael.
It sort-of hangs together as a theory …
