I talk about the Demon Dean character being somewhat inevitable.
The Supernatural fans know how much the demonic Dean storyline was foreshadowed, going all the way back to Season 3’s “Dream A Little Dream Of Me” when Dean is subject to dreaming of his worst nightmare which is himself, as a demon. At the time, this was because Dean had sold his soul for Sam’s life and was heading to hell where he would eventually turn into a demon. At the end of this episode, Dean reveals to his brother that he doesn’t want to die or go to hell, something that he had strangely not said before suggesting that he really does fear himself turning into a monster.
In the earlier seasons, fans were focused on Sammy’s journey to the dark side, with Azazel’s plan for his special psychic children, which included Sam, and Ruby manipulating him into drinking demon blood. This went as far as Sam’s eyes turning black in Season 4’s “Lucifer Rising”. I’m sure most viewers imagined that, out of the two brothers, Sam would be the one to turn bad which continuously overshadowed the fact that Dean went to hell for 40 years and tortured dozens of souls when he was there. These actions seem a lot more demonic that Sam’s actions which was to use his dark powers purely to kill Lilith.
After Dean is rescued from Hell, the show continues: Dean manages to lose his brother to Lucifer, live taunted by nightmares for a year without Sam, have to see his brother go through a soulless and then mental state, lose his beloved, Lisa, and therefore any chance of a normal family, and have one of his best friends, Castiel, lie and betray him and then die. Then it was purgatory where Dean felt, worriedly, free and almost at peace when he was made to kill tons of monsters every day, constantly on the run for a whole year whilst his brother had abandoned him for a real life. He then had to try and comfort his dying brother whilst he was going through the trials to close the gates of Hell and then, after stopping Sam from completing his mission, manipulating and tricking him into being possessed by a “psycho angel”. Kevin’s death, Dean felt, was completely his fault, stating how he’ll “burn for that” implying being sent to Hell yet again which, I’ll remind you, is his worst fear. The Mark of Cain was only the icing on top of this mentally scarring, emotionally damaged, self-loafing piece of cake that was years in the making.
My theory is that Dean’s demon future is not only due to the Mark of Cain but also due to hell, and purgatory where it appeared that Dean had finally found himself and found where he feels most happy and alive. During purgatory he had a focus and a goal which was freedom, returning to his brother which always seems like his ultimate purpose in life, and he achieved that goal. Most of the time when Dean aims for something he gets it. He set out to kill and succeeded to kill Azazel, Ruby and Dick Roman, the most notorious villains of the Supernatural world, mostly for revenge. Season 9 upped this violence and bloodlust in Dean using Abaddon, who Dean claimed he knew he would take down due to the confidence and power the First Blade and the Mark of Cain gave him.
Before burdening Dean with the mark, Cain warns him to which Dean brushes it to the side explaining how he needs to kill Abaddon, damning the consequences. However, because of his past life events, I believe that Dean already knew what the consequences were. Ever since the nightmare of himself as a demon, Dean could have known, all along, that someday he would turn into the thing he hunted. This may explain why, in Hell, he gave up resisting the torture and began torturing other souls himself, because he knew that it was almost part of his destiny to become a demon.
There is also the obvious similarity between Dean and Cain, who also sold his soul for his brother, Abel, which caused him to wreck havoc on earth in Lucifer’s name, like Dean did in Hell. Cain also found somebody who loved him for who he was no matter what sins he had committed; Colette. This seems similar to Dean and Lisa’s relationship as she also understood that Dean had to hunt and kill because it was who he was. (Although, Destiel fans may alter this part of the theory slightly)
Cain evidently became such a terrible demon because of his love for his brother and Dean obviously protects his brother as well, ever since he was a child and carried Sam out of the fire that the demon, Azazel, started to kill their mother. Heavy analysts could even say that Azazel’s fire that caused Dean to make that first action to protect his brother symbolises Abel first talking to Lucifer as Sam was subject to demon blood that day, beginning the whole demonic storyline. Both Abel and Sam turn to Lucifer in some way and Cain and Dean both strive to protect their blood, Dean spending all of his life trying to save Sam from the demon blood which would inevitably turn him into Lucifer’s vessel. This could also mean that, through talking to him, Lucifer was trying to make Abel his vessel but Cain stopped him which would explain why Sam is able to be possessed by Lucifer, as both brothers are descendants of Cain and Abel.
Despite both Cain and Dean’s promise to their partners Colette and Lisa, they both go back to killing and hunting. Although Lisa didn’t die because of Dean, like Colette was killed by Cain, she was stabbed and, after being healed by Castiel, was blanked of all memory of Dean. Dean felt this was necessary for Lisa and her son, Ben, to return to their lives, believing that without him they would be better off. However Dean still technically lost the person who he is shown to love the most, just like Cain.
As Crowley explains in the last episode of Season 9,“Do You Believe In Miracles?”, Cain killed himself because he didn’t want to turn into the thing the blade was making him. He failed as the mark turned him into a demon. This is so similar to Dean’s story as he died and became a demon due to the mark keeping hold of him. Even though Dean did not literally kill himself, in my opinion, I think it was very close to suicide. As I said before, Dean normally succeeds in killing the things he sets out to kill. Metatron was one of these bad guys and, as well as Metatron being too powerful for him, Dean knocked out Sam so that he wouldn’t get in the way of the fight. Dean could have done this because he knew that Metatron was too powerful and that he was going to die, wanting not to be saved by Sam and to accept it instead of becoming something he didn’t want to be. There is also the fact that Metatron is an angel as powerful as God, so, although Dean was doing the right thing by trying to kill him, he was technically on the opposite side to angels, going against the new God, therefore fighting for Hell and for Lucifer.
All of this suggests that Dean knew what was coming and it could have just been because he was noticing the changes in himself but it could be that Dean knew this was going to happen to him at some point. Ever since going to Hell, even though he was rescued before he turned into a demon, it had a massive affect on the rest of his life and trying to make a family with Lisa was his way of trying to fight his own dark destiny. When that went to pot, and when Dean spent his time in Purgatory, he finally accepted his fate and when Cain gave him the mark I think Dean knew exactly what he was getting himself into.