The Last Straw
Disclaimer I do not own Supernatural
Pain consumed Sam Winchester as the angel was ripped from his body. Physical pain worse than anything he had ever felt while a member of the land of the living, pain on par with hell, though not the cage. (Thank God for that, he didn't think that he could endure more of that kind of pain again.)
After the initial shock and subsequent mind numbing pain Sam felt something strange taking place in his head. At first they were fuzzy, but after a few seconds they came back in a rush; His memory.
'Oh. My. God...' Sam's mind was in chaos, his brother had tricked him into becoming possessed, was the only thought that could gain any type of traction. Everyone knew that Sam would forgive Dean anything; any remark, any beating, any words, any betrayal. But after countless verbal beatings and physical blows, after being replaced time and again at the drop of a hat, after being; accused, beaten down, forgotten, abandoned, and tricked, after all of this had been done to him and with zero to no remorse.
Sam had had enough. This was the last straw, the final how do you do, the final betrayal of his trust, the final let down. Throughout everything Sam had always had faith in Dean, even when they fought or Sam thought Dean was dead wrong, Sam had faith that Dean would try to do what was good what was right. But know?
Sam was in crisis, he had never thought that anything would ever challenge his faith in his brother, without this fundamental belief Sam didn't know anything. His brother could betray him so completely and utterly, what else could he do that Sam had always taken for granted that he wouldn't. In that moment Sam realized that he couldn't trust Dean or have that same faith that he had in him even in the midst of the apocalypse, he just couldn't.
Dean's hypocritical behavior couldn't be tolerated any longer. He should have realized that Dean would never change that "amazing" part of himself, he should have known better. After all this time Sam should have seen that every single time Dean would pat him on the shoulder after punching him, or every time Dean would walk out after verbally filleting him, that this would only ever end with one thing. Sam's death, not his physical or emotional death no, his spiritual and mental death. He couldn't keep making excuses for Dean, even if it was a part of him now, even when the hits keep coming Sam will always find a way to forgive Dean, well no longer. Because with that one choice Dean made to trick Sam into becoming possessed he did what no one, not even Lucifer could have done.
He broke Sam.
It was the last straw.