He knows what is being hidden behind whispered words and hushed tones. The glances that Dean shoots him from across the room. The moments when his own brother pretends to be asleep just to allow him those moments of veiled secrecy. What he's doing, what he's convinced himself is the only way, isn't something that Dean wants to hear about. It's not something that belongs in the pages of Chuck's books and it's not something that he can even say out-loud. It's implied each time that it's brought up. He tells Ruby, 'I need it.' and Ruby replies, 'It's yours.' They never speak of the actual act, not in terms that anyone else could disapprove of.
The events that are ahead, they're a weight on his shoulders that pushes down with every ounce that they could possibly put to him. It's not just revenge anymore, it's an answer. A solution to something that he feels only he has the ability to provide.
He lashes out.
Fists striking to his brother's features.
The spatter of blood that comes up to his eyes, that clings to his knuckles... it's not the same as his. Family doesn't stop at blood, it's something Bobby tells him often enough that it creeps up in his mind. Though, standing up, turning his back on his brother, knowing what he has to do - how far this has to go...
The pressure weighs on him and he can't do anything but continue on the path he's set himself onto.