When Dean touched Castiel on the shoulder, then patted his back, as he left the kitchen, he felt it, all of it. Cas had experienced human emotion. He was familiar with pain and sorrow and anguish. And he knew his friend was suffering. He followed Dean out of the room.
"Dean, may I ask you a question?"
When he got no answer, he asked again, believing that he needed permission. Although it was symbolic, he would not enter Dean's soul without acceptance.
Dean was all too familiar with Cas and his persistence. The question would be asked regardless of his wishes because, eventually, he would give in.
He took one more step before he stopped. He let his head drop forward keeping his arms at his sides.
He dreaded the subject of Cas' questions and felt he had very few answers to give. He turned, lifting his head, to face his friend. This would be his only indication of permission.
"When you killed Cain….."
"No, we're not talking about that. " Dean blurted out his words, cutting the question off. He threw his hands up, palms out, as if to push everything away.
"Please, Dean, just one question."
He slowly lowered his arms as his posture stiffened. It was the same stance one would take to control balance, to anchor oneself. But, Dean wasn't bracing himself against anything physical.
"When you killed Cain, how did it feel?"
Dean almost laughed. Feelings? He wanted to talk about his feelings?
He closed his eyes and when he opened them again, he had lowered his head just enough to be looking up into the angels eyes.
He spoke calmly and slowly. "I've killed before, Cas."
"But, this was a Knight of Hell. He was…."
Again, Dean interrupted him. He was breathing in quick short bursts. He lifted his head without shifting his gaze and said, " I've killed a Knight of Hell before, too."
"Ah, yes, Abaddon. And it was quite..." He paused as if carefully choosing the next word. "spectacular, as I understand."
"Spectacular" Castiel's monotone delivery sounded to Dean like he was listening to instructions on how to tie his shoes. The contrast made Dean more agitated and he spoke more harshly than he intended.
"What are you getting at, Cas? What do you want from me?"
Cas continued without changing his inflection.
"Just to know how you are feeling. I would imagine that dealing such a blow to the originator of the Mark, would affect you. Perhaps, even change its effect on you."
He was probably the most powerful force on the planet, maybe even beyond that, if he let it happen. And he was trying to reign it in like a dog on a leash. Dean hung his head. He could no longer look at the trusting face of his true friend. He could no longer answer his questions.