WARNING: THIS CONTAINS SEASON 10 SPOILERS.
Once the Mark was gone, Dean felt a wave of relief rushing through his body. He wasn't a monster anymore, he didn't feel that urge to kill every living soul. He was finally free. But there was one thing Dean couldn't stop thinking about. One thing that wasn't letting him go. It was guilt.
The moment he entered the bunker's library, memories overwhelmed him. Dean could feel a lump building up in his throat.
That night the Mark took over the control of his body and, more importantly, his heart and emotions. That night, when he was beating Cas up to death, he felt nothing but rage and emptiness. Dean thought he was able to fight the influence of the Mark, was able to resist it but he ended up being a heartless and emotionless shell. Only the pure light in Cas' blue eyes helped him to realise who he really was and what he was doing; otherwise the consequences could've been terrible and Dean wasn't sure he would've been able to bear inside that feeling of killing his best friend with his own hands.
"Dean?" a familiar voice called him.
At first Dean thought he had heard it inside of his own head but when he looked up his heart skipped a beat. Cas was there, in the library, taking a few uncertain steps towards Dean. His face was still covered in bruises even though a few days had already passed since that night. Apparently his grace still wasn't fully restored and wasn't able to cope with all his injuries at once. This view in front of him made Dean's heart ache so he just stared blankly back at Cas, unable to form any words.
"Dean?" Cas called him again. After everything the hunter had done to him, Cas still had concern in his voice and comprehension in his eyes.
All of it was enough to break down Dean's wall of self-control; he collapsed on the floor and tears started streaming down his cheeks. Cas was by his side almost immediately, "shh, Dean, it's okay, you're okay now, it's gonna be fine."
"No, Cas, it's not," Dean sobbed into Cas' shoulder, "I hurt you, I did this to you, it's all my fault. I could've just killed you!" Dean let out another sob, "I'm so sorry, Cas, I really am, and I always will be for the rest of my life, Cas, please, forgive me," Dean begged, not looking into Cas' eyes.
"Shh, Dean, it's okay. Of course, I forgive you. It wasn't your fault and there was nothing you could do to stop the Mark. Don't blame yourself, please, I'll be fine in a few days."
"But.." Dean finally lift his head to look at Cas, "but why didn't you fight back?"
Cas simply smiled in respond, "Because I couldn't have hurt you, Dean. I never did and I never will."
The sincerity in Cas' voice made Dean's guilt almost unbearable and he wasn't able to stop another sob.
They had been sitting there, in the library, for a very long period of time until Dean let out all his emotions while Cas was gently placing soft kisses on his temple and forehead, telling him that everything was going to be alright.
