Ruby thanking him was the very last straw. It was like she was rubbing his nose in the fact that yet again she had tricked them all.
And now he could see just how badly they had been duped. Sammy…Sammy sounded like a raving lunatic. He was going to save them all? How, by becoming a vampire? He had downed God knew how much of that bitch’s blood. It was even worse than if he had just slept with her, that would be an almost normal behavior but this…this was so far off the level of normal that it didn’t register anymore. Somehow, inexplicably, he heard his Dad’s voice in his head. He was going to have to save Sammy, or he was going to have to kill him. Well he had proven time and again that he didn’t have it in him to kill his baby brother, no matter how much everything said that he should. Even now he didn’t…
And Ruby was gloating. She had a lot to be smug about, from the way seeing Sam change was breaking Mary’s heart, to the disappointment Ben felt about his uncle failing to the pure anger John and Dean felt…maybe she was right, maybe this was his fault…but he knew how to end it. Though she was driving him even closer to the urge to torture than Alastair had, Dean resisted. Leaving her alive, even briefly, to torture her wouldn’t just mean losing all of the progress he’d made here, it would open them up to the chance that Castiel would find them and free her, and Dean wouldn’t let her live that long. He couldn’t’ hesitate if he wanted to cut Sam off at the source. He had to act now.
Again, there was no warning when Dean lunged forward. He didn’t ask Claire to let go of his hand, instead he just dropped her hand. He’d explain later. He didn’t give Castiel the Screw you he wanted to when Castiel threatened to make him sleep, because there’d be plenty of time to point out later how much the demon whore had brainwashed him. Explaining, reasoning, detoxing Sam…all of that had to wait until Ruby was dead.
He could only hope he got to that damn knife before Castiel had a chance to strike. Please, God. You don’t like this bitch anymore than I do. Let me get there before he can stop me. Just this once.