(note, this will likely be rambly, as it is not really meant for public consumption)

Five years between 2009, when Sammy said yes to Lucifer, and 2014, when (in Leap to Freedom) Lucifer was killed by Dean in a kamikaze raid on the Sanitarium where Lucifer was based.

What happened in those five years to make Dean into the cold, calculating camp leader that could so seemingly easily be willing to sacrifice those he was believed to be leading and protecting? What happened to make Dean into someone who could so willingly, easily sacrifice Castiel right along with that group? What happened to turn Castiel from angel of the Lord, fallen from Heaven but only because he'd believed that Dean was right, and therefore that following him was the right thing to do...into a hippy, junkie, and, most importantly, a so obviously broken human?

Losing Sam to Lucifer, to knowing that Sam went off on his own and voluntarily said yes to Lucifer and without even ever being able to know why utterly destroyed Dean. All he had left in the world at that point was Bobby, and Castiel. The knowledge that maybe, if he had said yes to Michael after all, he might have kept Sam from being taken and destroyed by Lucifer...that just ate away at him. But it was too late to change that now.

For Castiel, it was something entirely different. Oh, he, too, was devastated by what he could only see as Sam's betrayal. But, in the moment Lucifer gained his true vessel, the angels...were gone. Castiel and Lucifer were the only angels left to walk the Earth. And Heaven? There was no access to it, no sign it had ever even existed. It, and the angels, were just...gone. Castiel never learned what happened to them. Had they fled, knowing the battle was lost? Had they somehow all died? How had Heaven simply, seemingly, ceased to exist? Castiel had nothing left. Even his grace was slipping away, day by day. With no angels left, no Heaven, he was becoming human, permanently and irreversibly. There was only...Dean.

There was a window of peace, or as close to it as they could get with their knowledge of what was coming. Dean drove, and drove, and drove. He moved from place to place, hunting occasionally. The hunting became more and more infrequent, and he became more and more quiet and withdrawn. The only thing that really kept him going at all was the angel who stuck at his side, no matter what. Through rants and bouts of anger that erupted in violence, through binges with alcohol – at first – and then moving on to various drugs, through everything Dean tried to just forget and be numb...Castiel never left him.

But Castiel watched Dean, and he learned...and, occasionally, he even resorted to trying Dean's methods of escape and killing pain. Why not, right? His grace was waning. He was barely an angel anymore. The fact was that Dean often encouraged him, sometimes because the results were just funny, sometimes to have the company, and sometimes...because some part of him enjoying watching the angel fall further and further.

(Here, we have to resolve what you think might have happened with their relationship. In my head, they totally started getting it on.)

Within a year after Lucifer gained his true vessel, the Croatoan virus began to spread. Smaller areas were hit first, almost as though it were simply being tested in ways that could be easily contained. But the virus spread more and more quickly, and along with it the horror of the very zombie-like victims. Dean took to fighting the outbreaks where he could, until the hopelessness of it broke him further. Instead, he found an old, abandoned camp, and -along with Castiel and Bobby - hunkered down to do what Dean always did best in the face of overwhelming odds – survive. People would drift in, find them, and eventually they had an active camp, with Dean sort of default ending up in charge. Chuck would have come in somewhere during this time.

Blah blah. Bobby died somewhere in there, I always assumed due to Croats. That certainly would have broken Dean further, each terrible loss would have.

All of those losses changed Dean to the point he was focused on only one thing, destroying Lucifer. Killing the Devil was the entire point of his life.

Castiel, breaking himself because of watching Dean change and harden and become something cold and seemingly dead inside, started to turn back to the methods of escape and relief he learned from Dean early on. Drugs and alcohol. As he lost the last remains of his grace, he became addicted, the high replacing both Dean and the feeling of being an angel in his life.

As Dean grew colder, he began to treat Cas worse and worse, even being abusive (sometimes I think purely emotionally/mentally, sometimes I wonder if not physically sometimes, too) as he'd lash out at the former angel as a target because of three reasons. 1) He knew nothing he'd do would ever actually push Castiel away, that nothing he'd do could ever force Cas to leave him. Cas needed him too much. 2) Perhaps some part of him started to view Cas, as a former angel, to be at fault for what had happened since he was the last thing left of Heaven and the angels. 3) On some level he blamed himself for what Cas became, the helplessly addicted junkie constantly seeking a way to escape.

So we have a terribly twisted relationship between Cas and Dean by the time Dean discovers a way to finally really kill Lucifer, and plans the raid on the sanitarium. His plan absolutely relies on the party he leads being a distraction, and bait for the demons. Cas goes into it fully aware that Dean is willingly sacrificing them for his plan, and still goes along with it, because he's essentially helpless at that point to do anything but continue to follow Dean, even knowingly to his death. He even went into it hoping for that death, and suspected Dean knew that about him, too. Dean always knew everything about him.