He knew.
He had had his suspicions in the events leading up to his little bargain with Cain. He knew there was something wrong with him; that something was broken. Something had broken inside of him a long time ago. God knows how many times it had been drilled in his head, and it still didn't fucking stick.
"Look out for Sammy."
But then he took the mark. He fucked up. Again. He fucked up and took the mark and then he held the blade. He got himself in that stupid situation and then the First Blade was in his hand and– God, what had he done?
"Look out for Sammy."
He tried to explain. That bone was put in his hand and everything just... stopped. There was no right or wrong. No left or right. There was only hunting. It was like Purgatory all over again, but this was better. His fingers curled around that handle and everything goes quiet and it's calm– it's pure.
"Look out for Sammy."
He knew. He knew what was happening to him, and he couldn't bear to break that to his baby brother. Sammy would look at him with the same disappointment and shame in his eyes that he knew as a boy. The same cold stare he would get when he wasn't fast enough or wasn't efficient enough. Or when he didn't do his number one job.
"Look out for Sammy."
He knew. So he became reckless and he watched with the smallest twinge of relief as the piece of metal was shoved through his chest. It was done. He couldn't be a monster anymore. He wasn't poison. He wasn't a threat. He was gone– like he should have been a long time ago.
"Look out for Sammy."
But there's always a catch, isn't there?
"Open your eyes, Dean."
So he does. He opens his eyes and it's the best thing he's ever felt in his entire life. He doesn't feel a thing.
"Look out for Sammy."
He doesn't feel wrong or broken when he stands tall and mighty. He doesn't feel ashamed or guilty when his forearm glows with power.
"Look out for Sammy."
He doesn't feel anything when Sam sees him and stops dead in his tracks. He doesn't feel anything when his little brother's eyes gleam with tears and he sobs 'why?'
"Look out for Sammy."
He doesn't feel one goddamned thing when he sinks the blade into his baby brother's stomach.
"Look. Out. For. Sammy."
He looks at Sam writhing and screaming in pain underneath him. His boot slamming against his brother's body probably doesn't help, but he just shrugs it off.
"Look out for Sammy."
That was the number one rule. Was.
He grins devilishly with his brand new, shiny eyes.
He doesn't have rules anymore.