"Dean, stop!" he cries out, feeling the fingers curl too-tight around his arm.

Sam does nearly fall flat on his ass with that motion. Instead he hits the wall again, this time fully on his shoulder, and turns quickly at the danger -- the hunter instinct finally drives him and he kicks out aiming for Dean's stomach. But Sam saw this before, in his head; he knows Dean's gonna intercept, and then he'll close in, and then it won't be enough control and he'll gouge his teeth deep into Sam's esophagus --

'Hold still.'

So he draws the dead man's blood, ready for Dean to close that gap. And then when Dean inevitably bridges it, aiming for Sam's exposed long neck, Sam stabs him in the shoulder and injects him without hesitation. Because the other option is him dying, and as much as he's hurting and thinks maybe that's the point, Sam can't let it happen.